Thirty adult male mice were divided into three groups. The animals in group I were used as controls and drank only water during the entire period of experimentation. Group II animals drank water containing 1.5 g/100 mL zinc as ZnSO4, and group III animals received 2.5 g/100 mL zinc. After 3 wk supplementation with high doses of zinc, the animals were killed and the livers were removed and examined by electron microscopic techniques. After the supplementation period, the animals in groups 11 and 11 showed various degrees of degenerative changes in the hepatocytes, such as increased size and the presence of spaces and an abundance of lipid globules in the cytoplasm. The mitochondria showed a crystalline appearance, a diluted matrix, and dense aggregations. Some smooth endoplasmic reticulum tubules showed dilation and were filled with a dense substance. None of these changes were present in the group I control animals.