Female rats were exposed to a drinking solution containing either 650 or 1950 parts per million (ppm) of the organophosphate insecticide malathion for 14 days. Plasma samples were assayed for prothrombin time (PT), partial thromboplastin time (APTT), hematocrit (HCT), platelet count (PLT) and clot activation time for fibrinogen and coagulation factors, II, V, VII, VIII, X, and XII. Significant changes following 650 ppm malathion were noted for the fibrinogen, factors VII and XII. Drinking 1950 ppm malathion causes significant differences in factors II, VII, X and XII. The data strongly suggest that in the rat, malathion ingestion has a potent effect on blood coagulation.