Endosulfan, a chlorinated pesticide, is widely used to control various insect pests. Rats exposed to 1 mg and 3 mg endosulfan/kg for periods of 10, 30, and 60 days showed significant (P less than 0.05) inhibition of [3H]5-hydroxytryptamine (5-HT) uptake by platelet-rich plasma (PRP) in an ex vivo study. Rats treated with endosulfan (1 and 3 mg/kg) up to 60 days elucidated a marked inhibition of ADP-induced aggregatory responses of the platelets. Incubation of PRP with 10 microM and 100 microM endosulfan for 15 min at 37 degrees C also resulted in significant (P less than 0.05) inhibition of platelet aggregation in vitro. The paper discusses the use of rat blood platelets as a model for the study of neuro- and cardiovascular toxicity of endosulfan.