One day old mangrove rivulus (Rivulus ocellatus marmoratus) were exposed to 9 mg/l diethylnitrosamine (DEN) for 6 weeks, kept in water without DEN for an additional 18-20 months, then necropsied. Oncogene expression was detected by immunohistochemical staining of freeze-dried cryofixed livers. Positive controls for immunohistochemistry included tumors grown by injecting athymic nude mice with cell lines having known oncogene expression. Livers from 15 DEN-exposed fish contained 178 altered foci and neoplasms; 48% of these lesions over-expressed Ras, Myc, Fos, p53 or epidermal growth factor receptor (EGFR). Raf overexpression was not detected. Myc overexpression was positively correlated (P < 0.05) with smaller hepatocyte size in both hepatocellular neoplasms and in altered foci. Increased EGFR expression occurred primarily in inflamed lesions. Increased Ras expression in hepatocellular neoplasms was correlated with anaplasia, gamma-glutamyltranspeptidase activity and lesions that contained mixed acinar and trabecular profiles. Accumulation of p53 occurred more often in neoplasms than in altered foci and correlated with unusual cytoplasmic vacuoles. In hepatocellular neoplasms, Fos overexpression was correlated with increased cell diameter, nuclear pleomorphism, and enlarged nucleoli. Only 1/14 biliary neoplasms overexpressed an oncoprotein (Myc). None of the changes in oncoprotein expression were correlated with cell proliferation (bromodeoxyuridine staining). Although several of the correlations found in mangrove rivulus also occur in mammals, the general relevance of some of our findings can be determined only after they are confirmed in other species.