The present study was designed to evaluate the inhibitory effect of antidepressants on thrombin-induced phosphoinositide (PI) hydrolysis. Thrombin 5 units/ml induced a 100%-200% increase in platelet inositol phosphates (IPs) formation. This effect was inhibited in a dose-dependent manner by various heterocyclic antidepressants (IC50 40-170 mumol/L) The monoamine oxidase inhibitor, phenelzine, in concentrations up to 500 mumol/L, was devoid of inhibitory activity. The tricyclic antidepressants, (50 mumol/L) inhibited also thrombin-induced platelet aggregation by 32%-47%. No alteration in thrombin-induced IPs formation was detected in recovered major depressed patients (n = 15) maintained on clomipramine (75-150 mg/day). These results indicate that the heterocyclic antidepressants interfere with the thrombin-linked PI-signaling system. However, the inhibitory effect is achieved only in concentrations above the plasma concentrations obtained with therapeutic doses of these agents.