PAF causes dose dependent platelet aggregation of human platelet rich plasma or gel filtered platelets (GFP). The benzodiazepines alprazolam and triazolam, but not diazepam (1–10 μM), inhibit PAF induced aggregation but have no effect on aggregation induced by other platelet agonists such as ADP, epinephrine and collagen. The IC 50 for aggregation by PAF (4nM) in GFP is 1 μM for both alprazolam and triazolam. The mechanism for this inhibition was explored by studying the binding of 3H-PAF(0.08nM) to GFP in Tyrodes buffer containing albumin (0.35%), Mg ++ (1mM) and Ca ++ (0.5mM). GFP was incubated with different doses of the drug for 5 min prior to addition of 3H-PAF. Incubation was then carried out for 60 min at 25°C to achieve binding equilibrium, as previously established. Alprazolam and triazolam, but not diazepam, caused competitive displacement of 3H-PAF from specific binding sites of GFP. The IC 50 of alprazolam was 3.8 μM while that of triazolam was 0.82 μM. Lineweaver-Burk plots of 3H-PAF binding in the presence of inhibitor were also consistent with competitive inhibition. These results are consistent with the interpretation that the specific inhibition of PAF induced platelet aggregation by alprazolam and triazolam, respectively, is due to competitive inhibition of binding of PAF to its receptor.