A randomized double-blind study was carried out on 40 male patients requiring aorto-coronary bypass surgery. 20 patients received a constant dose of 8 ng kg-1 min-1 of prostacyclin (PGI2), beginning two minutes before extracorporeal circulation (ECC) and ending together with ECC. Compared to the placebo-treated patient group (n = 20), PGI2-treatment significantly reduced the ECC-induced release of platelet alpha-granule proteins, beta-thromboglobulin (1178 ng/ml vs. 1926 ng/ml) and platelet factor 4 (837 ng/ml vs. 1245 ng/ml) into plasma (mean of max. values). Furthermore the decrease of platelet counts during ECC was less pronounced in PGI2-treated patients. Application of PGI2 had no effect on the increase in thromboxane B2 (TxB2) plasma levels, which amounted to 0.6 ng/ml at the end of ECC. PGI2-treatment resulted in significantly elevated plasma concentrations of 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) (2.1 ng/ml) throughout the infusion off prostacyclin. 6-keto-PGF1 alpha plasma levels increased up to 1.2 ng/ml in the control group patients, indicating a stimulation of endogenous PGI2 formation during ECC.