The effects of sodium valproate (VP) and the sodium salts of its metabolites (2-en-VP, 4-en-VP, 4-OH-VP, 5-OH-VP and 2-propylglutaric acid) on gluconeogenesis have been examined using isolated rat hepatocytes. VP and its metabolites have a concentration-dependent inhibitory effect on gluconeogenesis from lactate: with increasing drug concentration, the lag in the onset of gluconeogenesis increases and the rate of glucose synthesis decreases. The toxic effect on glucose synthesis from lactate is very dependent on the metabolite used: the inhibitory effect is highest for VP and 4-en-VP, followed by 5-OH-VP, 4-OH-VP, 2-en-VP and finally by 2-propylglutaric acid. Thus, delta-dehydrogenation and omega-oxidation products in particular have a toxic effect on gluconeogenesis in isolated rat hepatocytes. Induction of the omega-oxidation pathway by enzymes inducers such as phenobarbitone may play a role in the eventual hepatotoxicity of VP. With glycerol the gluconeogenic substrate, the inhibitory effect of VP is also present, although to a smaller extent than with lactate. Glucagon abolishes the inhibitory action of VP.