The covalent binding of radioactivity to protein following administration of 14C-labeled analogs of valproic acid (VPA) and a hepatotoxic metabolite thereof, 2-n-propyl-4-pentenoic acid (delta 4-VPA), was investigated in male rats. Covalent binding occurred in a number of tissues, the level of binding being greatest to proteins in liver for each compound. Moreover, the binding of radioactivity from delta 4-VPA to hepatic macromolecules was higher than the corresponding value for VPA. When radiolabeled VPA and delta 4-VPA were incubated with rat hepatocytes, radiolabel again became bound to cellular proteins, the time-course of which suggested the existence of at least two underlying mechanisms. Thus, after initial rapid binding of both substrates, a secondary slow phase was evident, which favored binding of delta 4-VPA. Although phenobarbital pretreatment of rats had little effect on the covalent binding of either substrate to isolated hepatocytes, clofibrate pretreatment markedly enhanced the covalent binding of both VPA and delta 4-VPA to these cells. In contrast, the covalent binding of VPA and delta 4-VPA was suppressed strongly by 4-pentenoic acid, a potent inhibitor of beta-oxidation, but was not affected by metyrapone, an inhibitor of cytochrome P-450 activity. (-)-Borneol and 8-bromo-cAMP, two inhibitors of glucuronidation, acted to decrease the binding of both substrates, although this inhibition was evident only in the early stages of incubation. A similar effect was seen with valeric acid, the saturated analog of 4-pentenoic acid.(ABSTRACT TRUNCATED AT 250 WORDS)