A 10-wk lactation trial was conducted during the late summer and early fall of 2006 using 45 late lactation Holstein cows (199.7 " 66.3 DIM and 32.0 " 5.2 kg milk/d) to determine the effect of feeding supplemental mostly (85%) saturated (SAT) or more (50%) unsaturated (UNS) fatty acids on the performance and select metabolic measures. The first 2 wk of the trial served as a standardization period and all cows were fed the control diet without any supplemental fat other than whole cottonseed. At the end of wk 2, cows were blocked by parity and randomly assigned to one of three treatments with each block for 8 wk. Treatments included a control (no supplemental fat other than whole cottonseed), or the equivalent of 1 kg/d of mostly saturated or more unsaturated fatty acids. Dry matter intake, milk yield, and milk composition were similar among treatments. Average DMI, milk yield, and milk fat and protein percentage were 23.8 kg/d, 32.5 kg/d, 3.47%, and 3.23%, respectively. The BW and BCS of the cows were similar throughout the trial and all cows gained similar amounts of weight and condition. Concentrations of total cholesterol, HDL, and LDL in the blood were higher for cows fed diets supplemented with UNS compared with either control or SAT. Triglyceride and BUN concentrations were similar among treatments. Concentrations of NEFA were higher for UNS whereas insulin concentrations were higher for SAT than either control or UNS. There were no differences in internal body temperature among treatments but there was an interaction of treatment and time of day during wk 4 related to higher body temperatures for cows fed UNS at 0500 through 0530 compared with control and SAT and again at 0930 through 1030 compared with SAT. These results indicate that supplemental SAT or UNS did not significantly alter intake or performance of cows that have been through heat stress and were in late lactation; however feeding UNS did increase cholesterol and NEFA concentrations along with lowered insulin and tended to keep body temperature higher than either control or SAT-supplemented diets.