Using a prostacyclin in conjunction with standard heparin might limit the occurrence of post-extracorporeal circulation (ECC) thrombopathy and reduce the risk of haemorrhage inherent in this technique. For this reason, we studied the effect of the prostacyclin analogue Iloprost (ZK 36 374), a drug which is active in man when given orally with a biological half-life of 30 min, and devised a double-blind randomized trial to evaluate the potential benefit of Iloprost versus placebo in 2 groups of 15 patients (A: placebo, B: Iloprost). An infusion of the drug in incremental doses (up to 12 ng kg-1 min-1) was begun before starting the ECC and was stopped at the end of the cardiopulmonary bypass, at the time of protamine injection. Significant arterial hypotension was observed during ECC in two patients of the Iloprost group. Comparison between Iloprost and placebo groups showed that the mean number of platelets was not significantly higher in the Iloprost group 20 min after the ECC and during the early post-operative recovery period. Platelet aggregability was higher after surgery in the Iloprost group than in the placebo group. There was no significant difference in post-bypass bleeding between the two groups. Thus, Iloprost does not reduce the fall in circulating platelets observed during cardiopulmonary bypass, but it might help in preserving the platelet function. However, the potential usefulness of the drug is limited by adverse haemodynamic reactions.