After 4 days of acetysal treatment (160 mg/kg body weight orally), the following were established: a higher acute toxicity of acetysal, an inducing effect on amidopyrin N-demethylase and analgin N-demethylase activity and increases in cytochrome P-450 and cytochrome b5 content. Aniline hydroxylase activity decreased, thiopental sleeping time was prolonged and UDP-glucuronyltransferase activity was not changed. Dexamethasone, at a dose of 5 mg/kg body weight p.o. for 4 days, did not change acetysal acute toxicity but at a dose of 100 mg/kg i.p. increased it. Thiopental sleeping time was shortened by dexamethasone (100 mg/kg i.p.) but was not changed by dexamethasone at 5 mg/kg p.o., alone or in combination. Dexamethasone at 5 mg/kg increased analgin N-demethylase and UDP-glucuronyltransferase activities, did not change cytochrome P-450 content and decreased aniline hydroxylase activity. The combination with 5 mg/kg dexamethasone increased the activity of amidopyrin N-demethylase, analgin N-demethylase and UDP-glucuronyltransferase and decreased those of amitriptyllin N-demethylase and aniline hydroxylase and cytochrome P-450 content. Ethylmorphine N-demethylase, benzphetamine N-demethylase, NADPH-cytochrome c reductase and glutathione S-transferase activities were not affected significantly by acetysal, dexamethasone or their combination. Hepatic carboxyl esterase was depressed by dexamethasone (5 mg/kg) and was increased by the combination. Lipid peroxidation was not changed by dexamethasone (5 mg/kg) but was decreased by acetysal and the combination.