The effects of malachite green (MG) and phenobarbitone (PB) were compared on the development of pre-neoplastic lesions during N-nitrosodiethylamine(DEN)-induced hepatocarcinogenesis in male Wistar rats. Rats were administered 200 p.p.m. DEN in drinking water for a period of 1 month. After an interval of 2 weeks the animals were given either MG (25 p.p.m.) or PB (500 p.p.m.) in drinking water for 2.5 months. The effects were monitored on the basis of the morphological appearance of the liver, histological pattern, gamma-glutamyltranspeptidase (GGT)-positive foci, total GGT activity and the induction of glycogen-deficient islands. Both MG and PB were found to enhance liver carcinogenesis to a significant extent when compared with either their corresponding controls or animals given DEN alone. The enhancing effect of MG at 25 p.p.m. is comparable with PB at 500 p.p.m. An enhancing effect of MG on DEN-induced hepatocarcinogenesis in the rats was demonstrated.