Infusion of arachidonic acid through the guinea pig lung or the cat spleen causes a release of thromboxane A2 and prostaglandins, as measured by bioassay. After incubation of human platelets with arachidonate similar metabolites are formed, as demonstrated chromatographically. Infusion of imidazole (50-75 microgram/ml) through the lung or spleen specifically inhibits thromboxane A2 production and diverts the pathway to the prostaglandins, mainly prostaglandin F2alpha. In human platelets imidazole causes a dose-dependent inhibition of thromboxane A2 formation (ID50 5.5 X 10(-4) M). This inhibition is accompanied by a dose-dependent increase in prostaglandin F2alpha. Since thromboxane A2 induces platelet aggregation and is a potent vasoconstrictor, diversion of pathways to prostaglandins with opposite or less potent action might be of relevance in the treatment of cardiovascular diseases.