Fatty acid composition of brown, as well as white, fat of the rat wasanalyzed. Oleic, linoleic and palmitic acids accounted for more than 80 per-cent of the total fatty acids in brown fat. The proportion of linoleic acidwas markedly higher and that of palmitic acid considerably less in the brownfat of female rats than in that of male rats. Although differences weresmall, palmitoleic, myristic and lauric acids were less in female brown fat.In white fat the sex differences were small. Total saturated fatty acids weresignificantly less in the males than in the females in both brown and whiteadipose tissues. The sex differences observed in normal rats disappeared 2weeks after gonadectomy.In brown fat of rats exposed to cold, oleic acid decreased progressively, while linoleic acid and to, a less extent, stearic acid increased. The propor-tion of palmitic acid was reduced 24 hours and 1 week after the cold andpalmitoleic acid progressively in the cold. The percentage of total satu-rated fatty acids decreased 24 hours after the cold, but tended to increase thereafter.Changes in the fatty acid composition of white fat of rats exposed tocold were small. The only acid to show a significant decrease was palmiticacid.Starvation for 5 days induced a relative decrease of palmitic acid andan increase in stearic acid in both brown and white adipose tissues. A decrease in myristic acid was observed only in brown fat and there was anincrease in linoleic and linolenic acids in white fat. In both adipose tissuestotal saturated fatty acids decreased significantly, but the changes weremore pronounced in white fat.