Previous studies have demonstrated hepatic cytochrome P-450-dependent biotransformation of organic nitrates. We assessed whether this biotransformation resulted in the formation of an activator of guanylyl cyclase using the 100,000 x g supernatant of rat aorta as a source of crude enzyme. Incubation of aortic supernatant with rat hepatic microsomes and glyceryl trinitrate (GTN) resulted in concentration-dependent increases in guanylyl cyclase activity provided that the incubations were performed anaerobically and that reduced nicotinamide adenine phosphate was added. Cysteine-dependent activation of guanylyl cyclase by GTN was greater under anaerobic compared to aerobic conditions. Guanylyl cyclase activation by GTN was increased using hepatic microsomes from phenobarbital-treated but not beta-naphthoflavone (BNF)-treated rats and was decreased when microsomes from cimetidine-treated rats were used. The hepatic microsome-dependent activation of guanylyl cyclase by GTN was inhibited by in vitro treatment of microsomes with carbon monoxide, SKF 525A, metyrapone and cimetidine, but not by ranitidine. The sensitivity of isolated rat aorta to the relaxant effects of GTN was increased under low oxygen conditions or when aortae were obtained from phenobarbital- or beta-naphthoflavone-treated rats. Treatment of rats with cimetidine did not affect GTN-induced relaxation. The vascular biotransformation of GTN was increased greater than 3-fold when performed anaerobically, and this increase was prevented by pretreatment of the tissues with carbon monoxide. Together, these data provide strong evidence for the involvement of hepatic cytochromes P-450 in the formation from GTN of an activator of guanylyl cyclase (presumably NO or some closely related compound), and suggest that at least a portion of the vascular biotransformation of GTN is mediated by hemoproteins.