The objective of this experimental study was to examine the pathological effects of the intramuscular administration of gentamicin to growing chickens. Two experiments of 35 days´ duration were conducted simultaneously. In both experiments White Leghorn cockerels of 6 weeks of age were randomly divided into 4 groups and gentamicin was administered intramuscularly to the birds on days 1 and 3 at a dose of 10, 50, 100, and 0 mg/kg body weight, respectively. Birds administered 50 and 100 mg/kg gentamicin showed depression, diarrhea, increased water intake, and lower body weights. Mortality was 0%, 26%, and 100% in the 10, 50, and 100 mg/kg gentamicin groups, respectively. Kidneys and livers of the 50 and 100 mg/kg groups were swollen, friable, and hemorrhagic. Microscopically acute tubular necrosis was observed in the kidneys and fatty change of hepatocytes in the liver. Gentamicin administration resulted in a significant decrease in serum albumin and a significant increase in urea nitrogen and creatinine concentrations.