The effect of phenobarbital, a known inducer of drug-metabolizing enzymes, on hepatic glucose-metabolizing and NADPH-producing enzymes was evaluated in obese and lean female Zucker rats. Plasma glucose was slightly reduced and plasma insulin increased after one week of phenobarbital therapy. The induction clearly increased the activities of liver glucokinase in lean but not in obese animals. Hepatic glucose-6-phosphate phosphohydrolase activity, on the other hand, was reduced after phenobarbital, especially in obese rats. Thus phenobarbital affected the activities of hepatic enzymes modulating the intermediary metabolism in several ways: liver glucose phosphorylation was increased and glucose output reduced after enzyme induction in lean and obese Zucker rats, respectively. Phenobarbital also increased the activities of glucose-6-phosphate dehydrogenase and malic enzyme in lean animals indicating an increased NADPH generation.