Nitric oxide gas is a potent inhibitor of platelet aggregation, with an IC50 of 3.6 microM for rabbit platelets. Since the NO effect is mediated via increased cGMP, this in vitro study was undertaken to test the hypothesis that selective phosphodiesterase (PDE) inhibitors might enhance aggregation inhibition at lower NO concentrations. Because the cAMP-selective PDE III and the cGMP-selective PDE V are prominent in platelets, milrinone, a PDE III inhibitor, and zaprinast, a PDE V inhibitor, were tested alone and in the presence of NO for their effect on aggregation. Aggregometry was performed on rabbit platelet-rich plasma following addition of ADP as agonist. Milrinone alone gave an IC50 of 12.4 microM. With each agent set to give suboptimal inhibition of aggregation, the combination of milrinone (3-16 microM) and NO (2-10 microM) produced a greater effect than either agent alone. Zaprinast exhibited no effect on aggregation in concentrations up to 160 microM. However, adding zaprinast to 2 microM NO, which alone reduced aggregation approximately 30%, produced a marked synergism in the inhibitory effect up to and including no observable aggregation. These results indicate that elevation of either cAMP or cGMP is sufficient to inhibit platelet function. The platelet cAMP concentration appears high enough to be inhibitory when degradation is suppressed by milrinone. However, basal cGMP levels must be increased by NO before the zaprinast effect is observed.