L-Arginine and Pycnogenol. Both L-Arginine and Pycnogenol have good track records in the research for increasing endothelial Nitric Oxide, which is the prime chemical involved in erections.  Several studies have shown that supplemental L-Arginine increases blood flow, even in healthy individuals.  Furthermore, a well-done study shows that these can significantly cure and often cure erectile dysfunction (impotence). Nitric Oxide relaxes the veins of the penis allowing blood to flow in which obviously is critical for an erection. (Viagra, Cialis and Levitra work on Nitric Oxide for example.) By the way, you bodybuilders may be interested to know that Arginine is used to make creatine as well.
NOTE: Not all studies have shown L-Arginine in a positive light as far as being a promoter of Nitric Oxide. For example, one study of healthy males showed no increased blood flow from 20 g/day of Arginine, which is a very large dosage . Similarly disappointing results were found in patients in patients with heart failure as well.  It is much more safe and effective to Boost Nitric Oxide Through Food and Drink.
Pycnogenol is a Nitric Oxide producer or stimulator. (Several foods are.) L-Arginine is the precursor for Nitric Oxide in the body and for Nitric Oxide Synthase, the enzyme that crates Nitric Oxide, and thus can boost Nitric Oxide indirectly in several ways.  The idea that some researchers had was that Arginine could provide the building blocks for Nitric Oxide and then Pycnogenol would take that substrate and pump out more Nitric Oxide. One study had already showed that 50 men given 5 g L-Arginine per day doubled their urinary Nitric Oxide secretion.  However, that’s quite a bit of L-Arginine and so, I presume, the researchers wanted to find a safer approach using Pycnogenol.
Pycnogenol is a Nitric Oxide producer in its own right. Arginine also boosts Nitric Oxide directly by being its precursor. One study showed that 50 men given 5 g L-Arginine per day doubled their urinary Nitric Oxide secretion.  If you put the two together, reasoned one set of researchers, you ve got a potentially powerful solution to erectile dysfunction. And they reasoned correctly: their study of men with erectile dysfunction  showed very significant improvement in the ability to achieve an erection though a combination of 1.7 G L-Arg per day along side 2X40 mg doses of Pycnogenol. Steven Lamm, in his book The Hardness Factor, recommends a dose of L-Arginine of 3 g per day. He chose this value because it would approximately double what a male would consume through an average diet. (By the way, L-Arginine also increases sperm counts for those of you struggling with pregnancy-related issues.)
Italian researchers found that L-Arginine supplements actually helped blood sugar metabolism and insulin sensitivity of obese, Type II diabetics.  Of course, elevated blood sugar and insulin sensitivity is not just a problem of obese diabetics but is the curse of many on a Western diet . Those on a Western Diet also need the increased Nitric Oxide output in to help protect their cardiovascular system AND erections.
You may also be familiar with a nice side benefit of L-Arginine: the stimulation of growth hormone release. But keep in mind that this only occurs when ingesting fairly high levels of L-Arginine. Syracuse researchers found that 7 g/day of growth hormone increased growth hormone response.  And when coupled with weight lifting, an even greater growth hormone response was achieved. Other studies list doses (to achieve growth hormone output) between 8-12 g/day. Again, I recommend caution with such high doses. (See Side Effects below.)
What is a good way to decrease arginase activity? Well, one recent animal study shows that caffeine does just that. Rats given a relatively small dose had significantly reduced arginase  and one reason is probably caffeine’s increase in Cyclic AMP. This is just one study and is on animals, but it probably explains why caffeine and L-Arginine can be a winning combination. (Be careful not to have so much caffeine that it disturbs your sleep!)
I should also point out that anther recent study actually investigated arginase inhibitors – there are drugs that will, like caffeine, inhibit arginase expression – for sexual disorders.  Their conclusion? The study stated, “Accordingly, arginase inhibition can enhance NO-dependent physiological processes, such as the smooth muscle relaxation required for sexual arousal: administration of arginase inhibitors in vitro and in vivo enhances erectile function and engorgement in the male and female genitalia. Therefore, arginase is a potential therapeutic target for the treatment of sexual arousal disorders in men and women”. But this is cutting edge and has not yet resulted in any actual treatment protocols.
However, you can get in on the ground floor with a combination of pycnogenol (or other dietary Nitric Oxide boosters specified in my Erectile Dysfunction Dietary Program), L-Arginine and caffeine. This is definitely a winning combination.
Side Effect #2: If you have a heart condition, work with your doctor before taking Arginine or any supplement. One Journal of the AMA study [Vol. 295 No. 1, January 4, 2006] reported that researchers gave 153 heart-attack survivors 9 g daily of L-Arginine. Six participants on the L-Arginine died, pushing the researchers to actually halt the study. Keep in mind that thousands and thousands of people have taken L-Arginine without incident and that the study individuals had heart problems. Also, keep in mind that this is a fairly high dosage of L-Arginine: I would recommend more in the 2-3 g range. Regardless, it underscores the need to get the advice of a medical professional.
Side Effect #3: L-Arginine taken at higher doses is alleged to cause higher body ammonia levels leading to potential herpes and/or shingles outbreaks. This appears to be ameliorated through the consumption of cruciferous vegetables such as brocolli, cauliflower and cabbage which are high in indole-3-carbinol (I3C). I3C interferes with the way that shingles and herpes viruses reproduce.
Also, if you do decide to take L-Arginine supplementally, it is probably more effective on an empty stomach. It can compete with other dietary components, such as L-Lysine, and so is probably better absorbed just by itself.
1) J Sex Marit Ther 2003 May-Jun; 29(3):207-213
2) Amer Jour Physio Endocrinol Metab, 291:E906-E912, 2006
3) Jour Applied Physiol 101:848-852, 2006
4) BJU Int,1999,83:269-273
5) J Nutr 2009 Feb;139(2):230-7
6) Molec Cell Biochem 244:125-8 (2003)
7) Acc Chem Res, 2005 Mar,38(3):191-201
8) Intl J of Cardiology, Apr 2007, 116(3): 300-308
9) J Nutr. 2005 Feb;135(2):212-7
10) Am J Cardiol, 2004 Apr 1, 93(7):933-5
11) Vasc Med, 2003 May,8(2):77-81
12) Journal of Cardiovascular Pharmacology, Jul 1996, 28(1):158-166, “Effects of In Vivo and In Vitro L-Arginine Supplementation on Healthy Human Vessels”
13) J Am Coll Cardiol, 1996; 27:1207-1213, “Dietary supplementation with L-arginine fails to restore endothelial function in forearm resistance arteries of patients with severe heart failure”