The development of renal tumors was examined in rats given 1000 p.p.m. N-ethyl-N-hydroxyethylnitrosamine (NEELA) and then 30 000, 10 000 or 3000 p.p.m. of trisodium nitrilotriacetate monohydrate (Na3 X NTA X H2O) in their food. The incidences of renal tumors in rats treated with NEELA for 2 weeks and then 30 000, 10 000 and 3000 p.p.m. Na3 X NTA X H2O for 30 weeks were 100, 86 and 22%, respectively, while they were 16 and 0% in rats treated with NEELA or 30 000 p.p.m. Na3 X NTA X H2O alone. Na3 X NTA X H2O at 30 000 or 10 000, but not 3000 p.p.m., also increased the size and number of renal tumors in rats treated with NEELA, and Na3 X NTA X H2O at 30 000 p.p.m., but not 10 000 or 3000 p.p.m., also induced hydronephrosis in rats irrespective of whether they were treated with NEELA. Thus, Na3 X NTA X H2O has a dose-related effect on the kidney.