Viagra or Arginine for Nitric Oxide production?
         Like the drug sildenafil citrate (Viagra), L-arginine is thought to enhance the action of nitric oxide and in turn relax the muscles surrounding blood vessels supplying the penis. As a result, blood vessels in the penis dilate, increasing blood flow, which may help maintain an erection.
         L-arginine is used to make nitric oxide -  Viagra blocks an enzyme called PDE5, which destroys nitric oxide . Unlike Viagra, L-arginine is taken on a daily basis for erectile dysfunction.
                            [Full-range squats with a relatively heavy weight will also likely increase blood flow to the area - Vince]
        Sexual stimulation causes the brain to activate the release of nitric oxide from L-Arginine in the penis & clitoris. Nitric oxide then promotes an increase in the molecule cyclic GMP (cyclic guanosine monophosphate). Elevation of cyclic GMP gives rise to the erection response. Viagra does not increase cyclic GMP production; it only inhibits the breakdown of existing cyclic GMP in the body. Viagrašs disturbance of cyclic GMP action throughout the body can have adverse effects in many different tissues.   info links here and here