Five herds with 240 cows were used to determine the response to feeding 6 g/d of supplemental niacin during the summer months. Diets consisted of corn silage, hay crop silage, hay, or pasture as the forage and concentrate. Cows in each herd were paired by lactation number and stage of lactation and randomly assigned to either a control or niacin group and fed niacin once daily for 84 d (July 3 to September 25). Cows averaged 143 d postpartum with a total of 121 and 119 cows on control and niacin groups. Using milk weight and composition prior to initiating the trial as a covariate, least squares means for all cows for control and niacin were milk, 24.5, 25.4 kg/d; milk fat, 3.72, 3.70%; milk fat, .91, .94 kg/d; milk protein, 3.35, 3.34%; and fat-corrected milk, 23.5, 24.3, kg/d. Yields of milk and fat-corrected milk were significantly higher for niacin-supplemented cows. Cows in first lactation and second lactation and greater had a similar response to niacin. Cows producing greater than 34 kg milk produced 2.4 kg more milk with supplemental niacin. Results indicate that feeding 6 g/d supplemental niacin during the summer increased yield of milk and fat-corrected milk but had no effect on milk composition.