Rats were administered parathion (8 mg/kg, oral gavage) followed 20 min later by menadione (40 mg/kg, ip). Sixty min after parathion treatment, rats were sacrificed. A, Effects of menadione on signs of parathion toxicity. Data are mean ± SE, n = 3–4. *Significantly different from control (p < 0.05) and †significantly different from parathiontreated (p < 0.05). B, Effects of menadione on parathion metabolism in liver, brain, and serum. C, Effects of menadione on carboxylesterase activity in rat livers and sera following parathion treatment. D, Effects of menadione on cholinesterase activity in rat brains following parathion treatment.