Caffeine and arginine in RPM are effective in the blend. It is useful, though, to recall that arginine is not necessarily nitric oxide. L-Arginine is the substrate for two enzymes, arginase and nitric oxide synthase. These enzymes compete against each other for the arginine metabolic pathway. The enzyme arginase converts arginine to ornithine and urea. Nitric oxide synthase converts arginine to nitric oxide, the potent vasodilator. A higher expression of the arginase enzyme leads to higher ornithine production, but less nitric oxide production. Now enters caffeine (methylxanthine caffeine). Caffeine inhibits arginase activity. It does this by inhibiting the so-called adenosine receptors, leading to higher levels of free adenosine. Now, adenosine, adenine, inosine, and uric acid, are known to be competitive arginase inhibitors. (BCAAs also demonstrate arginase-inhibitory effects, by the way). Furthermore, caffeine and caffeine-related compounds, such as theobromine, also produce arginase inhibition by raising norepinephrine and cAMP. Anyway, the caffeine-triggered arginase inhibition produces an elevated expression of the nitric-oxide synthase enzyme. This means even more nitric oxide can be produced from arginine, leading to stronger vasodilation (and pump). This is just a reference to caffeine and arginine. If one adds the nitric-oxide-synthase promoting effects of OPCs (in RPM), and the synergy from the rest of the compounds in the blend, then RPM's potency becomes easier to fathom.