The carcinogenic potential of triethanolamine was examined in F344 rats. Triethanolamine was dissolved in distilled water at levels of 0 (control), 1, and 2%, and groups of 50 males and 50 females were given these doses ad libitum as drinking water for 2 yr. The dose levels in females were reduced by half from wk 69, because of associated nephrotoxicity. A variety of tumors developed in all groups, including the control group, and all tumors observed were histologically similar to spontaneous tumors in this strain of rats. No statistically significant increase of the incidence of any tumor was observed in the treated groups of both sexes by the chi-square test. In this study, however, there was an increase in nephrotoxicity, which appeared to have an adverse effect on the life expectancy of the treated animals, especially of females. Therefore, an age-adjusted statistical analysis on incidences of main tumors or tumor groups of both sexes was also done by methods recommended by Peto et al. (1980). The result showed that a positive trend (p less than 0.05) was noted in the occurrence of hepatic tumors (neoplastic nodule/hepatocellular carcinoma) in males and of uterine endometrial sarcomas and renal-cell adenomas in females. These tumors, however, have been observed spontaneously in this strain of rats, and their incidences in the control group of the present study were lower than those of our historical controls. These results may indicate that a positive trend in the occurrence of these tumors is not attributable to triethanolamine administration. Increased incidence of renal tumors in the female high-dose group may have been connected with renal damage. Histological examination of renal damage observed in the treated groups, especially in the female high-dose group, revealed acceleration of so-called chronic nephropathy. In addition, mineralization of the renal papilla, nodular hyperplasia of the pelvic mucosa, and pyelonephritis with or without papillary necrosis were also observed. Thus, it is concluded that under these experimental conditions triethanolamine is not carcinogenic in F344 rats but is toxic to the kidneys.