Urinary nitrate (NO3) is the stable end product of nitric oxide, which is formed, in turn, from a guanidino nitrogen of arginine. We have conducted two experiments, each in four healthy adult men receiving a low nitrate diet for 7-10 days, to investigate the in vivo conversion of arginine to nitrate.In the first study [guanidino-'5N2, 5,5-2H2]arginine was given on day 7 via a primed continuous intravenous infusion for 8 h. In the second study, the labeled arginine was given for 8 h by the intragastric route on day 7 and by the intravenous route on day 10. Measurement of 1IN03 output in urine collected for 24 h beginning at the time of the arginine tracer infusion revealed a more extensive transfer of 15N when the arginine tracer was given intragastricly. From the comparative labeling of 1IN03 after administration of the tracer arginine via the intragastric and intravenous routes, we estimate that 16% ± 2% of the daily production of nitrate arises from the metabolism of dietary arginine that is taken up during its "first pass" in the splanchnic region. Hence, nitric oxide production occurs, to a measurable extent, in this area in healthy subjects, raising the question as to how various pathophysiological states might alter the relations between exogenous and endogenous sources of arginine as precursors of NO-and the relative contributions made by various organs to whole body (NOB) NO3 formation. These results also raise important questions about the use of nitric oxide synthase inhibitors in animal and human studies.The endogenous synthesis of nitrate by mammals (1, 2) was demonstrated to occur via oxidation of trivalent nitrogen (i.e., amino nitrogen) and to be greatly stimulated by an endotoxin challenge (3). Studies by Marletta and coworkers (4, 5) with murine macrophages disclosed an enzymatic pathway that involved the oxidation of a guanidino nitrogen of L-arginine to nitric oxide (NO-) and its subsequent oxidation and excretion as urinary nitrate (e.g., refs. 6 and 7). Numerous other investigations concerning problems of cardiovascular physiology, neuronal signaling, and endotoxic shock have resulted in the discovery of a large family of NOsynthase enzymes (NOS; EC 1.14.23.-). These NOS include both constitutive and inducible forms and may be membrane bound or cytosolic depending on cell type; the physiological roles of these enzymes have been recently reviewed (8-11). In addition, reports have appeared recently on the isolation and cloning of some of the key genes encoding NOS (12-14), which revealed close sequence similarity to cytochrome P450 reductase (15) and also with functional characteristics of a P450-type hemoprotein (16). Thus, given the apparently large number of isoforms of NOS, their varying distribution in human tissues, and their multiple mechanisms of activation, it is of importance to define the in vivo relationships between arginine metabolism and urinary nitrate in man.In an earlier study, Leaf et al. (6) gave a large oral bolus dose (85 mg-kg-' of body weight) of [15N]arginine t...