With a auto-balanced terbidimeter, the effects of ligustrazine, aspirin and betahistine on platelet aggregation were studied in 45 patients with acute ischemic stroke. The adopted parameters were: 1) the maximal aggregation (Amax), 2) the maximal aggregation velocity (Vmax), 3) the effective disaggregation rate in 5 minutes (DA 5), 4) the inhibition rate of aggregation given by drugs (IR), and 5) the concentration causing 50% inhibition of aggregation (IC 50). The results indicated that these three drugs could inhibit platelet aggregation both in vivo and in vitro. Aspirin could promote the aggregated platelets disaggregation in vivo and ligustrazine, in vitro. Regarding the effects on inhibiting platelet aggregation in vitro, ligustrazine was the most noticeable among the 3 drugs and aspirin was more effective than betahistine. The IC50 of ligustrazine, aspirin and betahistine were 0.568 mg/ml, 1,286 mg/ml and 1.722 mg/ml respectively. The authors considered that all three drugs possessed anti-platelet effects but they showed some differences among them.