The hepatocarcinogen acetamide, in single doses of 100 and 400 mg/kg b.wt., was shown to act as an initiator in a dose-dependent fashion in rat liver using the Solt-Farber method. Acetamide and its putative metabolite N-hydroxy-acetamide did not cause liver necrosis in single dose experiments. Acetamide showed no evidence for genotoxicity in tests for mutations in Salmonella typhimurium, for DNA damage in rat hepatoma cells or for DNA repair in isolated rat hepatocytes. In contrast, N-hydroxy-acetamide displayed genotoxic activity in all 3 test systems. Neither acetamide nor N-hydroxy-acetamide induced transformation of primary Syrian hamster embryo cells or gave evidence of inhibition of metabolic cooperation in V79 cells. Radiolabelled acetamide and N-hydroxy-acetamide were not bound covalently to proteins in the presence of various metabolic activation systems (microsomes plus NADPH or xanthine/xanthine oxidase, cytosol or cytosol plus acetyl CoA or proline plus ATP). N-Hydroxy-acetamide was cytotoxic to monolayers of isolated hepatocytes at concentrations above 2.5 mM. This cytotoxicity was increased after diethyl maleate treatment, but N-hydroxy-acetamide did not deplete cellular glutathione. A HPLC system was developed for the separation and quantification of acetamide, N-hydroxy-acetamide and acetic acid. No significant excretion of N-hydroxy-acetamide or acetic acid in the urine could be demonstrated after treatment of rats with 100 or 1,000 mg/kg b.wt. of acetamide. The underlying mechanism for the observed initiating effect of acetamide is obscure.