A recent study from our laboratory demonstrated that cyclosporine (CsA), a prototype immunosuppressant, enhanced the growth of carcinogen-induced enzyme altered foci in rat liver, suggesting that CsA may stimulate development of hepatocellular carcinomas. In the present study, we examined (i) whether CsA accelerates development of hepatocellular carcinomas in experimental animals, (ii) whether CsA stimulates the proliferation of resting hepatocyte in vivo and (iii) whether CsA modulates the production of growth factors implicated in liver cell growth, hepatocyte growth factor (HGF), transforming growth factor alpha (TGF alpha) and transforming growth factor beta 1 (TGF beta 1). Foci of hepatocytes, positive for glutathione S-transferase placental form were induced in male F344 rats by a single dose of diethylnitrosamine followed by 7 weeks promotion by a choline-deficient diet. The animals were then divided in two groups, and subsequent development of hepatocellular carcinomas was compared in rats fed a basal diet or a basal diet containing 0.015% CsA. Development of hepatocellular carcinoma was accelerated in the rats maintained on a CsA diet. Feeding a CsA diet as the sole treatment, for 2, 4 and 10 weeks induced significant increases in liver weight, and resulted also in an enhanced incorporation by hepatocytes of 5-bromo-2-deoxy-uridine. Serum levels of glutamate-oxaloacetate transferase, glutamate-pyruvate transferase and lactic dehydrogenase were not altered by feeding a CsA diet. Northern Blot analyses of the expression of HGF, TGF alpha and TGF beta 1 mRNAs in the liver showed similar patterns of expression between rats fed a basal diet and a CsA diet. The levels of HGF mRNA were not altered in the lungs and kidneys of rats fed a CsA diet. These results indicate that CsA stimulates rat liver cell proliferation in vivo without inducing liver cell necrosis, and that this effect may contribute to accelerated development of hepatocellular carcinomas in rats fed a CsA diet. As previously observed with BR 931, a hypolipidemic peroxisome proliferator, stimulation of liver cell growth by CsA did not entail changes in the production of HGF, TGF alpha or TGF beta 1.