Technical Abstract: Effects of feeding pregnant dams dietary fat during the last 55 d of gestation on cold tolerance of neonatal calves sired by bulls with (Piedmontese) or without (Hereford) muscular hypertrophy genes was determined. F1 heifers of the respective breeds received diets containing either 2.2% (Low Fat; LF) or 5.1% (High Fat; HF). Safflower seeds containing 37% oil with 79% linoleic acid were the supplemental fat source in isocaloric-isonitrogenous diets. At parturition, calves were separated from their dams, jugular-vein cannulated and at 5 h of age, placed in a 0 deg C room for 140 min. Rectal temperatures and blood samples were obtained at 10 and 20 min intervals. Blood was assayed for cortisol and glucose. Calves from HF dams had higher rectal temperatures (P<.05) than calves from LF dams, and the HF calves maintained higher rectal temperatures throughout cold exposure (P<.01). Cortisol and glucose concentrations in calves from HF dams suggested these calves were under less stress and had more (P<.01) glucose available for metabolic heat production than did calves from LF dams. Calves sired by Piedmontese-cross bulls maintained higher rectal temperatures and had higher cortisol and glucose concentrations than did Hereford-cross calves. Feeding the pregnant dam supplemental fat during late gestation increased heat production in the neonatal calf and potentially could increase calf survival; calves predisposed to muscular hypertrophy may have a different ratio of shivering vs non-shivering thermogenesis due to differences in body composition or relationships among uncoupling proteins.