In order to investigate whether different 'promoters' have the same qualitative and/or quantitative effects on rat hepatocarcinogenesis, 0.05% of phenobarbital (PB), 0.05% of dichlorodiphenyltrichloroethane (DDT), 0.5% butylated hydroxytoluene (BHT) and 0.1% of nafenopin (NAF) were chronically administered in the diet to rats previously submitted to an initiation by diethylnitrosamine and a selection with 2-acetylaminofluorene plus CC14. The animals were killed after 3, 6 and 14 weeks of 'promoters' administration to analyse their effect on premalignant lesions. The quantitative analysis of the gamma-glutamyltransferase positive lesions indicates that as compared to a control group receiving a basal diet after initiation and selection, PB, DDT and BHT enhance the development of these lesions whereas NAF inhibits it. Rats were also killed after 22 weeks of administration to analyse the incidence and the yield of liver cancer. As compared to the control group, PB, DDT and surprisingly NAF enhance the development of liver cancer whereas BHT does not. This suggests that the effect of potential 'promoters' should be analysed on cancer development rather than on premalignant lesions.