The objectives of this experiment were to determine long-term responses to supplemental fat (from whole soybeans and liquid animal fat) and to determine whether the supplementation of nicotinic acid would enhance milk protein content or yield. From wk 4 through 43 postpartum, 44 multiparous Holstein cows (10 to 12 per treatment) were assigned to one of four dietary treatments: 1) control, 2) control plus 12 g/d of nicotinic acid, 3) supplemental fat, and 4) supplemental fat plus 12 g/d of nicotinic acid. The dry matter intake of cows did not differ among dietary treatments. Yields of milk, solids-corrected milk, and 3.5% fat-corrected milk were increased by nicotinic acid; the yield of fat-corrected milk during wk 4 to 25 was increased by supplemental fat. Contents of crude protein (CP) and true protein in milk were less for cows fed diets supplemented with fat or nicotinic acid; casein content was decreased by nicotinic acid. Intake of net energy for lactation was greater for cows fed supplemental fat; energy balance was greater during wk 4 to 25 for cows fed diets supplemented with fat. Body condition score and body weight were less when nicotinic acid was added to the control diet than when it was added to the diet supplemented with fat. Supplemental fat increased the concentration of nonesterified fatty acids (NEFA) in plasma; nicotinic acid increased NEFA when it was added to the control diet but decreased NEFA when it was added to the diet supplemented with fat. Nicotinic acid did not prevent the decrease in milk CP content that was induced by dietary fat, but it did increase milk yield and tended to increase the yield of milk CP.