Phenytoin was fed to rats and mice in their diet for 6 wk. The dosage was about 80 mg/kg. In rats, phenytoin treatment had no effect on tissue and body weights and had a minimal effect on the hepatic mixed-function oxidase (MFO, EC 1.14.14.1) system. Plasma ascorbic acid levels were higher in the phenytoin group than in the controls, but tissue levels and the rate of ascorbic acid synthesis were similar in the two groups. Also, copper concentration in liver and kidney was significantly higher in phenytoin-treated rats than in controls. Iron, zinc and manganese levels were unchanged in comparison to control values in liver, kidney, heart and brain. In contrast to the results with rats, phenytoin treatment in mice resulted in a lower body weight and a clear induction in the hepatic MFO system compared to that in controls. Phenytoin treatment resulted in higher liver ascorbic acid tissue levels than in controls. Liver copper and kidney zinc were lower and liver and kidney calcium and bone iron were higher in phenytoin-fed mice than in controls. This study shows that for both species phenytoin feeding affected ascorbic acid and tissue trace element metabolism. The clinical significance of these findings with regard to the nutritional status of the human patient undergoing treatment with phenytoin needs to be considered.