In the rat the hepatotoxicity of bromobenzene is greatly enhanced by the introduction of a cyano group adjacent to the bromine (i.e. o-bromobenzonitrile). Epoxide metabolites of these aryl halides are believed to be the actual toxic species, but epoxides of the latter compound also possess a second site of chemical reactivity not found in epoxides of bromobenzene, i.e. a Michael acceptor group formally related to acrylonitrile. Because these epoxides have never been isolated or synthesized for direct evaluation of their toxicity we have determined the toxicity toward isolated rat hepatocytes of a series of cyclohexene and cyclohexadiene derivatives containing epoxide and/or alpha, beta-unsaturated nitrile functional groups. Simple epoxides and unsaturated nitriles were much less toxic than bromobenzene itself, even if both groups were present in the same molecule. However, alpha, beta-unsaturated epoxides were found to be 2-3 times more toxic than bromobenzene, and at least 10 times more toxic than their saturated analogs, which is consistent with their relatively greater chemical reactivity. It is unlikely that Michael acceptor metabolites account for the increased toxicity of o-bromobenzonitrile relative to bromobenzene.