Using fluorescence optical and electron spin resonance spectroscopy, we have investigated the production of superoxide by bovine endothelial nitric oxide synthase (NOS). In contrast to neuronal NOS, the heme moiety is identified as the exclusive source of superoxide production by endothelial NOS. Thus, calmodulin‐mediated enzyme regulation affects production of nitric oxide and superoxide simultaneously and inseparably. The balance between the nitric oxide/superoxide reaction pathways may be shifted by addition of exogenous heme‐specific agents, such as tetrahydrobiopterin. Our results have direct relevance for the pathophysiology of atherosclerosis.