Twenty-six lactating Holstein cows (90 d of lactation) were blocked according to milk production, parity, and days of lactation for assignment to one of two dietary treatments. Diets included a control diet with no supplemental niacin and a diet supplemented with increasing concentrations of niacin (12, 24, or 36 g/d per cow over three consecutive 17-d periods. Cows were housed in a covered free-stall barn and were fed and milked twice daily. Mean maximum air temperatures and temperature-humidity indexes were 28.5, 31.4, and 25.2 degrees C and 79.6, 85.1, and 75, respectively, for the three periods. Rectal temperature was measured with a rectal probe, tail and rump temperatures by infrared thermometry, and respiratory rate by visual observation. Measurements were made daily at 0800, 1600, and 2200 h. Rectal temperature was not affected by treatment. Comparison of skin temperatures for control cows and cows fed niacin showed higher temperatures at the tail (34.0 vs. 33.7 degrees C at 0800 h; 35.1 vs. 34.8 degrees C at 1600 h, respectively) and rump (34.1 vs. 33.7 degrees C at 0800 h; 35.3 vs. 35.0 degrees C at 1600 h, respectively) for control cows during period 1. No differences in thermal responses were observed during period 3. Niacin did not significantly increase milk production but decreased skin temperatures during periods of mild or severe heat stress.