A rat model was used to study renal function after unilateral kidney irradiations. The left kidney was irradiated with a single dose (6-14 Gy), 2 equal sized fractions (10-16 Gy total dose) or 4 equal sized fractions (12 to 21 Gy total dose). At regular time intervals after treatment, the left kidney function was assessed with the use of 99mTc-Dimercaptosuccinic acid for a period of about 1 year. It was found that renal function declined in a dose- and time-dependent manner. The threshold dose for detecting functional impairment was 8 Gy for 1 fraction, 12 Gy for 2 fraction and 15 Gy for 4 fraction irradiations, demonstrating the sensitivity of the isotope tracer test used in these experiments. Retreatment of the rats with a single i.p. dose of 5 mg/kg cis-Dichlorodiammineplatinum (II) (cis-DDP) given at about 1 year after X ray exposure revealed an important relative decrease in the function of the irradiated compared to the unirradiated kidney. Reductions in function of 11 to 70% (depending on radiation dose and schedule) compared with control values were observed at 11 weeks after drug injection. These results demonstrate that a previously irradiated kidney is more sensitive to a subsequent treatment with cis-DDP than the contralateral hypertrophied kidney in the same animal.