OBJECTIVE To study the effects of 6 h inhalation of aerosolized prostacyclin (PGI2) on platelet function. METHODS In a prospective, double-blind, randomized study, 28 patients scheduled for elective cardiac surgery requiring cardiopulmonary bypass (CPB), received either 0.9% sodium chloride (n = 8), PGI2 5 microg x ml(-1) (n = 10) or PGI2 10 microg x ml(-1) (n = 10) as an aerosol for 6 h postoperatively. METHODS Cardiothoracic intensive care unit at a university hospital. METHODS All patients were studied immediately after surgery during mechanical ventilation and sedation. The PGI2 solutions or saline were administered with a jet nebulizer. RESULTS Bleeding time and chest tube drainage were measured. Blood samples for platelet aggregation, thrombelastography (TEG) and analysis of coagulation parameters and the stable prostacyclin metabolite 6-keto-PGF1alpha were obtained immediately before inhalation and after 2, 4 and 6 h of inhalation. After 6 h of PGI2 inhalation, regardless of administered dose, there was a lower rate of platelet aggregation and a lower maximal increase in light transmission in response to adenosine diphosphate (ADP) than in the control group. The TEG variable reaction time (R) was prolonged after 4 and 6 h of inhalation in the PGI2 group receiving 10 microg x ml(-1). There were no differences between groups with respect to bleeding time and chest tube drainage or any of the other variables examined. CONCLUSIONS Inhalation of PGI2 for 6 h in patients after cardiac surgery is associated with impaired platelet aggregation detected by in vitro techniques, with no in vivo signs of platelet dysfunction.