The effects of molybdenum (Mo) supplementation in the drinking water at the levels of 0.1, 0.5, 1.0, 2.0, 5.0 and 10.0 mg/l on the hepatic trace element concentrations and enzyme activities of female Sprague-Dawley rats were studied. The mean hepatic Mo concentration increased significantly in the rats supplemented with 0.1 mg Mo/l as compared to the nonsupplemented rats, but a further significant increase did not occur until the supplementation level reached 5-10 mg Mo/l drinking water. Hepatic copper concentration of the group given 0.1 mg Mo/l and hepatic iron content of the groups given 0.1 or 0.5 mg Mo/l were significantly higher than those of the other groups. The hepatic xanthine dehydrogenase/oxidase activity was not significantly affected by Mo supplementation. The hepatic sulfite oxidase (SOX) activity of the group given 0.1 mg Mo/l was significantly higher than that of the nonsupplemented group. The SOX activities of all the other supplemented groups were at a significantly different level intermediate between the first two. The hepatic superoxide dismutase (SOD) activity was significantly higher in the group given 0.1 mg/l than in the other groups. These results indicated that molybdenum enzymes and SOD might not be participants in previously reported anticarcinogenic activity of Mo, as supplementation at the level of 0.1 mg/l had been observed to be inefficacious in inhibiting N-nitrosomethylurea-induced mammary tumor incidence.