Background: The intravenous infusion of sodium nitroprusside is widely used as a means of producing deliberate hypotension in a variety of clinical situations. However, sodium nitroprusside reported to inhibit platelet aggregation. So we studied the effects of sodium nitroprusside on platelet function in patients undergoing intracranial aneurysm surgery with isoflurane anesthesia. Methods: Platelet rich plasma from the patients receiving sodium nitroprusside was studied for aggregation in response to adenosine diphosphate, epinephrine and collagen. Maximum aggregation rate and maximum aggregation time were evaluated from the samples collected at pre-sodium nitroprusside infusion, 30min and 90min after sodium nitroprusside infusion, respectively. At the same time, bleeding time was measured. Results: The mean maximum aggregation rate of adenosine diphosphate, epinephrine and collagen at pre-sodium nitroprusside infusion decreased significantly 30min and 90min after sodium nitroprusside infusion, respectively(P<0.05). But the maximum aggregation time showed no significant change. Prolongation of bleeding time was not observed after sodium nitroprusside infusion. Correlation between the total sodium nitroprusside dose delivered and the maximum aggregation rate of adenosine diphosphate, epinephrine and collagen were significant (r=0.797(P<0.05), r=0.732(P<0.05) and r 0.737(P<0.05)). Conclusions: In situation where sodium nitroprusside is administered for deliberate hypotensive anesthesia during intracranial aneurysm operation, the platelet aggregation was inhibited by sodium nitroprusside. However, bleeding time was not prolonged. (Korean J Anesthesiol 1996; 31: 217-223)