ZK 36374 (Iloprost), a stable prostacyclin analogue, was administered to 6 healthy volunteers for 2-hour periods, with dose rates increasing from 0.5 to 2 ng/kg/min within that time. At these doses, which did not give troublesome side effects clinically, there was significant inhibition of ex vivo platelet aggregation responses to ADP and collagen. There was some rebound platelet hyperaggregability in all subjects, occurring between 1 and 2 h after termination of the infusion; this was of minor degree and was not associated with any clinical problems.