Sodium nitroprusside is reported to inhibite platelet aggregation. We studied the effects of sodium nitroprusside on platelet aggregation function were in 30 adult patients undergoing spine surgery with enflurane anesthesia. Platelet-rich plasma from 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 serially from the samples collected at post-induction (pre-sodium nitroprusside infusion), 30 min and 90 min after sodium niroprusside infusion. The mean maximum aggregation rate of adenosone diphosphate, epinepherine and collagen at post-induction decreased significantly 30 min and 90 min after sodium nitroprusside infusion (P 0.05),but maximum aggregation time showed no significant changes. The correlation between total sodium nitroprusside dose delivered and maximum aggregation rate of adenosine diphosphate, epinepherine and collagen were r=-0.54 (P<0.05), r=-0.706 (P<0.05) and r=-0.794 (P<0.05). In summary, in situation in which sodium nitroprusside is administered during deliberate hypotensive anesthesia, the anesthesiologist must be aware of the potential for the inhibition of platelet aggregation.