In this work, we explored the role of cyclic nucleotides in modulating parameters of the Na/H antiport in human platelets. Sodium nitroprusside and iloprost, as well as cyclic nucleotide analogues, were used to raise cellular levels of cAMP and cGMP. Cyclic nucleotides reversed the thrombin-evoked alkaline shift in cytosolic pH set point and the activity of the Na/H antiport, concurrently with attenuation of thrombin-induced rise in cytosolic free Ca. No effect of cyclic nucleotides was observed in platelets not treated with thrombin, or platelets subjected to phorbol 12-myristate 13-acetate. cAMP did not reverse ionomycin-induced changes in the parameters of the Na/H antiport. Collectively, these observations indicate that cyclic nucleotides modulate the Na/H antiporter in human platelets through their effect on thrombin-evoked changes in cytosolic free Ca. Presumably, this effect holds for other agonists which stimulate phospholipase C, raise cytosolic-free Ca, and activate the Na/H antiport through protein kinase C dependent and protein kinase C-independent mechanisms.