Pulegone and menthol, components of peppermint oil, were investigated in rats. The substances were administered by gavage for 28 days at 0, 20, 80, 160 mg pulegone and 0, 200, 400, 800 mg menthol/kg body wt./day, respectively. At the two highest doses, pulegone induced atonia, decreased blood creatinine content, lowered terminal body weight and caused histopathological changes in the liver and in the white matter of cerebellum. For menthol at all dose levels a significant increase in absolute and relative liver weights and vacuolisation of hepatocytes was found. No sign of encephalopathy was observed in rats given menthol. The no effect level for pulegone was 20 mg/kg body wt./day and for menthol less than 200 mg/kg body wt./day.