Mature rabbits were injected intra-muscularly into the hamstring (HS) muscles with doses of 1.66 mg Cu/kg/day, as cupric acetate, in aqueous solution. Similar doses of Na acetate were injected into control rabbits. Skeletal (HS) and cardiac (MYO) muscles were examined by scanning electron microscopy, transmission electron microscopy and a microanalyzer and energy dispersive X-ray (MEDX). Na-injected rabbits showed slight myofibrillar lesions in HS after four injections, and mitochondrial inclusions in MYO were noted after two or seven injections. No Cu was found with MEDX. Cu-injected rabbits showed that the severity of lesions of myofibrillar degeneration and mitochondrial inclusions in HS increased with the number of injections and days the rabbits had been on experiment. After seven days on experiment signs of muscle regeneration were seen. Cu was found first to localize in the nucleus of HS after one day, thereafter it localized in the mitochondria after three and seven days. MYO lesions were found in the mitochondria after six or seven days after Cu-injection. With MEDX less Cu was found than had been found in HS. This Cu first localized in the nucleus of MYO after one day and then disappeared from the tissue altogether.