Administration of cyclophosphamide at a dose which is lethal to 10% of control athymic nude mice resulted in sudden death within 3 h in all mice that had been pretreated with the glutathione synthesis inhibitor L-buthionine-SR-sulfoximine. In Fischer 344 rats pretreated with L-buthionine-SR-sulfoximine, the cyclophosphamide dose producing 100% acute toxicity was lowered from 500-150 mg/kg; cardiac monitoring revealed ventricular fibrillation to be the cause of death. These and additional studies reported demonstrate that cytoplasmic glutathione is an important protectant against the cardiac and skeletal muscle toxicity of cyclophosphamide and indicate that such toxicity may be substantially increased by glutathione depletion. Since diet and many drugs (including cyclophosphamide itself) are known to affect glutathione levels, the present studies suggest that cardiac and skeletal muscle glutathione content is likely to be a clinically significant determinant of the frequency and severity of the adverse drug interactions and systemic toxicity sometimes observed during cyclophosphamide therapy.