Control, prilled fat (5% of ration DM), niacin (12 g/d), or fat and niacin treatments were fed to 39 Holstein cows beginning 17 d prior to expected calving through 15 wk postpartum to determine effects on hepatic lipid content, plasma ketone concentration, and lactation performance. Cows were blocked according to season of calving (cool = November 1 through April 1; warm = April 2 through August 1). Fat supplementation tended to increase milk yield but only for cows that calved in the warm season. Milk composition was not affected by treatments. Fat supplementation did not decrease BW loss in early lactation but increased rate of BW gain af ter 8 wk postpartum. Dry matter intake and glucose, nonesterified fatty acid, and beta-hydroxybutyrate concentrations in plasma were not different among treatments. Fat and niacin supplementation tended to increase hepatic total lipid and triglyceride content. Between 17 d prior to expected calving and 1 to 2 d postpartum, hepatic lipid content increased approximately 2-fold and triglyceride content increased 6- to 10-fold. Hepatic lipid and triglyceride contents were greater postpartum during the warm season than the cool season and were greater at 5 wk than at freshening during the warm season but lower at 5 wk than at freshening during the cool season. The cause of the dramatic increase in hepatic lipid and triglyceride content prepartum is unknown.