The antiaggregating agent prostacyclin (PGI 2) was infused into ten dogs during cardiopulmonary bypass (CPB) to minimize thrombocytopenia and platelet dysfunction. The animals were anesthetized, placed on mechanical ventilation and underwent thoracotomy. After heparinization with 300 u/kg, animals were assigned to control (n=5) or PGI 2 treated groups (n=5). Thoracotomy and then CPB decreased platelet numbers to below 30, 000/mm 3 (p < 0.05) and fibrinogen to less than 150 mg/dl (p < 0.05). PGI 2 at 100 ng/kg·min was infused for the 2 h period of CPB. PGI 2 infusion did not prevent these changes, but did prevent platelet serotonin release. In the control group after CPB, platelet serotonin fell from the baseline value of 1.11 μg/10 9 to 0.35 μg/10 9 platelets (p < 0.05). In contrast, PGI 2 treatment resulted in a serotonin increase to 2.27 μg/10 9 platelets (p < 0.05). Thromboxane B 2 concentrations of platelets and plasma rose during CPB (p < 0.05). Surprisingly, PGI 2 infusion accentuated this rise in platelet and plasma thromboxane B 2 (p < 0.05). These data indicate that during CPB, an infusion of PGI 2: 1) does not prevent thrombocytopenia; 2) increases platelet serotonin uptake despite, 3) an associated rise in platelet and plasma thromboxane B 2.