Dehydroepiandrosterone (DHEA) inhibits glucose 6-phosphate dehydrogenase (G6PD) activity and growth of preneoplastic lesions in various tissues, but its administration may also enhance tumorigenesis by genotoxic carcinogens. We have investigated in single preneoplastic liver lesions, induced in diethylnitrosamine-initiated rats by the resistant hepatocyte protocol, the mechanisms underlying these opposite DHEA effects. Administration of DHEA (0.45% in the diet) for 10 and 26 weeks and of its analog 16alpha-fluoro-5-androsten-17-one (FA, 0.25%) for 10 weeks, starting 4 weeks after initiation, induced an apparent decrease in the number of glutathione S:-transferase (placental) (GST-P)-positive lesions and an increase in lesion volume. DHEA administration for 38 weeks enhanced hepatocellular carcinoma multiplicity. Depending on the rise in the number of slowly growing, remodeling GST-P-positive lesions induced by DHEA and FA, overall DNA synthesis decreased slightly in these lesions at 14 weeks, but increased in uniform lesions. Labeling index (LI) in single uniform lesions at 14 weeks ranged between very low (not different from normal liver) to high (>10-fold normal liver). DHEA and FA induced broad increases in lesions with a high LI, which showed a higher number of cells overexpressing c-Ha-ras and/or c-fos than those with a lower LI. High G6PD activity was inhibited by DHEA and FA in only approximately 50% of preneoplastic lesions. These data indicate selection in rats subjected to long-term DHEA and FA treatments of a subpopulation of GST-P-positive cells with high growth and progression potentials. Overall effects of these compounds depends on the relative numbers of lesions in which inhibition of DNA synthesis can counteract their transforming effect.