After oral administration, ticlopidine specifically inhibits ADP-induced platelet aggregation, prolongs the bleeding time and prevents thrombosis in man. Its mechanism of action is not well known. Ticlopidine inhibits ADP-induced binding of fibrinogen to platelet glycoprotein GP IIb-IIIa but not shape change and increases deaggregation. Ticlopidine has no direct effect on the GP IIb-IIIa complex. We studied the effects of ticlopidine (500 mg/day for 8 days) in four healthy male volunteers on washed platelet aggregation induced by 5 μM ADP or thrombin (0.1 units/mL) and potentiated by 1 μM adrenaline (Adr), on basal and 1 μM PGE₁-stimulated cAMP levels and on elevation of cytosolic free Ca²⁺ concentration ([Ca²⁺]_i). We found that: (i) ticlopidine inhibits aggregation by ADP but not the potentiation by Adr of ADP-induced aggregation; (ii) ADP, Adr or thrombin decreases cAMP levels raised by PGE₁, an effect inhibited by ticlopidine only for ADP and not for Adr or thrombin; and (iii) Ca²⁺ influx and Ca²⁺ mobilization from internal stores were not affected. These results suggested that ticlopidine or a metabolite impairs the coupling mechanism of the ADP aggregation pathway at an unknown level.