BACKGROUND: Glucocorticoids inhibit hepatocellular proliferation and modulate the expression of oncogenes and tumor suppressor genes via mechanisms involving the glucocorticoid receptor. Glucocorticoids also produce a receptor-mediated inhibitory effect on both basal and hormone-stimulated expression of a newly discovered family of molecules important for shutting off cytokine action. We therefore hypothesized that inhibiting glucocorticoid receptors may disturb hepatocellular growth and apoptosis. Consequently, we investigated the effect of RU486, a potent antagonist of the glucocorticoid receptor, on basal levels of hepatocellular proliferation and apoptosis in male B6C3F1 mice. Furthermore, we evaluated the effect of this compound on cellular genes involved in the regulation of these important processes. RESULTS: Data show that treatment of male B6F3C1 mice with RU486 (2 mg/kg/d, ip) for 7 days dramatically inhibited liver cell proliferation by about 45% and programmed hepatocellular death by approximately 66%. RU 486 also significantly increased hepatic expression of the oncogenes mdm2 and JunB, while reducing that of the tumor suppressor gene p53. CONCLUSION: Exposure to RU486 may ultimately enhance the susceptibility of the liver to cancer risk by diminishing its ability to purge itself of pre-cancerous cells via apoptosis. This effect may be mediated through increases in the hepatic expression of the oncogene mdm2, coupled with decreases in that of the tumor suppressor gene p53. The decrease in hepatocellular proliferation caused by RU 486 may be related to effects other than its anti-glucocorticoid activity.