Effects of supplemental prilled long-chain fatty acids on lactation performance during heat stress were examined using eight multiparous Holstein cows in a replicated 4 x 4 Latin square design with 15-d periods. Cows were ruminally cannulated and were assigned randomly to one of four treatments in a 2 x 2 factorial arrangement of treatments. Factors were 0 or 5% supplemental fat and thermoneutral or heat stress conditions. Cows were housed in environmental chambers with thermoneutral conditions of 20.5 degrees C and 38% relative humidity for 24 h/d or heat stress conditions of 31.8 degrees C and 56% relative humidity for 14 h/d and 25.9 degrees C with 56% relative humidity for 10 h/d. Isonitrogenous diets (17% CP) containing 50% alfalfa silage and 50% concentrate were offered for ad libitum intake. Diets contained 1.64 or 1.83 Mcal NEL/kg DM. No diet by environment interactions were significant. Milk fat percentage (3.46 vs. 3.15%) and 3.5% FCM (31.5 vs. 29.2 kg/d) were higher for cows fed 5 vs. 0% fat. Dry matter intake, milk yield, and milk protein percentage did not differ between diets. Heat stress decreased DMI, milk yield, 3.5% FCM, and milk protein percentage but did not affect milk fat percentage. Results suggest that supplemental fat at 5% of diet DM enhances lactation performance similarly under thermoneutral and heat stress conditions.