This study was conducted to examine the effects of single dose enalapril (1.6 mg/kg) on platelet aggregation alone and after an intravenous bolus dose of the nitric oxide synthase (NOS) inhibitor N-nitro-L-arginine (L-NNA; 40 mg/kg). Blood samples were collected before and 1, 2 and 6 h after the administration of drugs in rhesus monkeys. Enalapril was administered via a nasogastric tube while L-NNA was administered by the intravenous route. Platelet aggregation was stimulated with adenosine diphosphate (ADP) in platelet rich plasma (PRP). Enalapril significantly inhibited platelet aggregation with a peak effect at 6 h. Upon administration of L-NNA, there was a significant increase in the percentage of platelet aggregation. This effect was partially antagonized by enalapril. The present study demonstrates that enalapril may possess antiaggregatory effects, which may be mediated via nitric oxide mechanisms.