The aim of this experimental study was to investigate the pathological effects of gentamicin in birds. Broiler chicks at 1 day of age were intramuscularly administered gentamicin at dose levels varying from 0 to 180 mg/kg. Clinical signs comprising of depression, decreased feed consumption, increased water intake, loose watery droppings and reduced body weights appeared in a dose-related manner in chicks administered 30 mg/kg or higher dose levels. Mortality was 0% and 20% in chicks given 40 mg and 50 mg/kg gentamicin. It increased in a dose-related manner and was 100% in 100mg/kg or higher dose groups. The LD50 calculated at total deaths in 5 weeks duration was 77.56 mg/kg. Kidneys and livers of chicks given 50mg/kg or higher doses of gentamicin were congested, enlarged and had hemorrhages on the surfaces. Microscopically kidneys exhibited acute tubular necrosis. Livers showed fatty change, vacuolar degeneration, necrotic areas and cellular infiltration around portal triads. Serum total proteins and albumin decreased while creatinine and ALT increased in chicks given 20mg/kg and higher doses. The no observable effect level (NOEL) of a single intramuscular administration of gentamicin in day-old broiler chicks was 10mg/kg body weight.