Abstract Male, Sprague-Dawley rats were weaned on postnatal day 17 to isocaloric diets in which fat supplied 10% (PWC group) or 65% (PWF group) of the available energy. Compared with rats allowed to wean spontaneously to maternal (low-fat) diet, the PWC group showed early increases in the activities and quantities of hepatic glucose-6-phosphate dehydrogenase (G6PD) and malic enzyme (ME). Although the response of G6PD was diminished in the PWF group, the high-fat diet did not suppress the rise in the activity or the amount of hepatic ME resulting from premature weaning. In contrast, the activities and amounts of both G6PD and ME were diminished when the high-fat diet was fed to 40-day old rats for 5 days. The activities of liver G6PD and ME were highly correlated with the quantities of immunochemically detectable enzyme proteins throughout the entire experiment.