The effects of acute and chronic ethanol administration on tumor progression and metastasis were studied in rat models of leukemia and breast cancer, respectively. Acute administration of 1.5-3.5 g of ethanol/kg body weight significantly reduced survival of rats injected with CRNK-16 leukemia cells in a dose-related manner. Acute administration of 2.5-3.5 g of ethanol/kg body weight, one hour before tumor inoculation, or chronic consumption of liquid diet containing ethanol for two weeks before and three weeks after tumor inoculation, significantly increased the number of lung metastases of MADB106 mammary adenocarcinoma. The ethanol-induced increase in the number of metastases was not correlated with plasma levels of corticosterone and was not altered by the opiate antagonist naltrexone. Incubation of spleen cells in vitro in the presence of ethanol, at concentrations comparable to those measured in the blood of ethanol-treated rats, significantly suppressed natural killer (NK) cell activity against MADB106 cells in a standard chromium-release assay and decreased the binding of effector to MADB106 tumor cells. However, neither acute nor chronic ethanol administration in vivo altered splenic NK activity against this tumor in the same in vitro assay, in which the ethanol would have been washed away. These results suggest that, in the presence of ethanol, tumor progression is facilitated. The possibility that this facilitation is related to ethanol-induced impairment of the normal tumoricidal interaction between NK and tumor cells is discussed.