The activity and submicrosomal distribution of alpha-glycerophosphate acyltransferase (GPAT) were studied in rats fed ethanol for 6 wk. GPAT activity was also measured in rats after 10 days of alcohol feeding, 22 days of phenobarbital administration, or 24 days on a high fat (71% of total calories) diet. After 6 wk of ethanol feeding, GPAT activity was increased 73% when expressed per milligram of protein and 133% when expressed per 100 g of body weight (P < 0.005). GPAT activity was more abundant in the smooth than in the rough microsomes of both control and ethanol-fed rats when expressed per milligram of microsomal protein and when expressed per gram of liver; the smooth microsomes accounted for most of the increased GPAT activity after ethanol. 10 days of ethanol feeding or 22 days of phenobarbital administration did not increase GPAT activity. Feeding a high fat diet for 24 days increased GPAT activity per milligram of protein to an extent similar to that observed after chronic ethanol administration. When expressed per 100 g of body weight, however, the increase was much greater after ethanol. The significance of these findings in vivo has not been elucidated. Increased GPAT activity might contribute to the persistence of alcoholic fatty liver and the development of hyperlipemia.