Five groups of male weanling rats were provided purified diets containing 15% by weight of either fresh or laboratory-heated corn oil (FCO, HCO) or fresh, laboratory-heated, or commercial pressure deep-fry peanut oil (FPO, HPO, PPO). Total weight gain, feed consumption, and feed efficiency were consistently greater for the FCO, FPO, and PPO groups. Although relative heart weights were unaffected, the HCO and HPO produced elevated liver and kidney weights. The dietary fats had no effect on the hematological status of the animals or the proportions of cells comprising the total leukocytes of the blood. Physical, chemical, and microscopic evaluation of the urine detected no pathologic conditions. Rats fed diets containing HCO or HPO demonstrated toxicity of thermally oxidized fats by the appearance of diarrhea, dermatitis, seborrhea, and hair loss. Histological examinations revealed injury of the thymus by all fat samples except the FCO; the liver was damaged by the HCO, HPO, and PPO, and the testes and epididymides by HPO and PPO. In the latter case there was complete cessation of spermatogenesis.