Tris(2,3-dibromopropyl) phosphate (TBP) was administered in the feed at one of two concentrations to groups of 55 male and 55 female inbred F344 rats and to 50 male and 50 female (C57BL/6N X C3H/HeN)F1 mice. The high and low dietary concentrations of TBP administered orally were 100 and 50 ppm for the rats, respectively, and 1,000 and 500 ppm for the mice, respectively. For each rodent type, 55 animals of each sex were used as contols. In both rodent types, renal epithelial tumors developed at incidences that were significant for male and female rats and mice that received the doses. These tumors included renal tubular cell adenomas and carcinomas that developed from the proximal convoluted tubular epithelium. Among female mice and rats, hyperplasia and/or dysplasia of the proximal convoluted tubular epithelium with or without cystic dilatation of the tubules and increase in the size of cell nuclei were dose dependent and recognized as preneoplastic and/or toxic lesions. The comparative histogenesis of renal tubular neoplasms was discussed.