It is generally accepted that the biotransformation of organic nitrates to an activator of soluble guanylyl cyclase (presumably NO) is a prerequisite for their vasodilator actions. The glutathione S-transferases (GSTs) mediate glyceryl trinitrate (GTN) biotransformation, but whether this results in guanylyl cyclase activation and relaxation of vascular smooth muscle is equivocal. We used electroporation of adherent cultured cells to deliver the membrane-impermeable GST inhibitor basilen blue (BB) into porcine kidney epithelial cells. This resulted in significant inhibition of GTN biotransformation because of a reduction in the formation of glyceryl-1,2-dinitrate, but not glyceryl-1, 3-dinitrate. In the 105,000 x g supernatant fraction of porcine kidney epithelial cells, BB significantly inhibited the formation of both GTN metabolites. Electroporation of porcine kidney epithelial cells with BB also inhibited GTN-induced cyclic GMP accumulation. This was caused in part by inhibition of soluble guanylyl cyclase by BB. To differentiate BB-mediated inhibition of the bioactivation of GTN from its inhibitory effect on guanylyl cyclase, inhibition of cyclic GMP accumulation induced by GTN and that induced by the spontaneous NO-releasing compound, t-butyl-S-nitrosothiol were compared. Maximum inhibition of cyclic GMP accumulation by BB was 80% and 40% with GTN and t-butyl-S-nitrosothiol as the stimulating compounds, respectively. These data suggest that GSTs mediate the biotransformation of GTN to an activator of guanylyl cyclase and support the contention that vascular GSTs participate in mediating the relaxant effects of organic nitrates.