Ethanol-fed rats showed significantly elevated plasma and liver gamma-glutamyltransferase activity, compared with rats fed carbohydrates isocalorically, or with untreated control animals. The activity of gamma-glutamyltransferase in liver was detected by means of both biochemical and histochemical methods. These data suggest that increased plasma gamma-glutamyltransferase activity commonly found in alcoholics can be related, at least in part, to a hepatic enhancement of the activity in the liver.