Progression of hepatic neoplasia was assessed in medaka (Oryzias latipes) following aqueous exposure to diethylnitrosamine (DEN). Larvae (2 weeks old) were exposed to 350 or 500 p.p.m. DEN for 48 h, while adults (3-6 months old) were exposed to 50 p.p.m. DEN for 5 weeks. Fish were maintained as long as possible to determine malignant potential of resultant neoplasms. A total of 423 medaka with 106 hepatic neoplasms were examined. There were marked differences in tumor prevalence between exposure groups including: (i) higher prevalence of hepatocellular carcinomas in medaka exposed as adults (100% of hepatocellular tumors in adult-exposed medaka were malignant, while only 51.5% of larval hepatocellular tumors were malignant); (ii) higher prevalence of biliary tumors in medaka exposed as larvae (46.4% of all tumors in larval-exposed medaka were biliary versus 8.1% in adult-exposed fish); (iii) higher prevalence of mixed hepato-biliary carcinomas in adult-exposed medaka (24.3%) compared with those exposed as larvae (3%). In addition, a unique hepatocellular lesion termed 'nodular proliferation' was only observed in adult-exposed medaka. The lesion was characterized by small size (50-300 microm), complete loss of normal tubular architecture and variable megalocytosis. Nodular proliferation was distinct from preneoplastic foci of cellular alteration and may represent microcarcinomas. There was a step-wise increase in mean diameter with age (days post-exposure) from nodular proliferation (174 microm, 17 days) to hepatocellular carcinoma (1856 microm, 62 days) and mixed carcinomas (3209 microm, 93 days) in adult-exposed medaka. Metastasis was observed with 19 neoplasms and tumors with the highest metastatic potential were hepatocellular and mixed carcinomas. The most common form of metastasis was trans-coelomic, followed by direct invasion and distant metastasis, presumably via the vascular route. Differences in tumor prevalence between exposure groups were believed to be the result of length of DEN exposure rather than age of fish at the time of exposure. In larval medaka with brief (48 h) DEN exposure, neoplasms are thought to be the result of dedifferentiation of hepatic cells, with slow progression of foci of cellular alteration to benign and then malignant tumors. In contrast, with adult medaka and prolonged (5 week) DEN exposure, neoplasms are believed to result from initiation of committed stem cells and formation of microcarcinomas ('nodular proliferation'), before progressing to larger hepatocellular and then mixed carcinomas.