According to the case records found in literature of the last ten years, a combined therapy with ionizing radiation dose and systematic application of cytostatic drugs leaded much earlier to nephropathy, if the kidney was situated directly within the ray path, than radiotherapy alone. Hitherto no systematic experimentations on animals have been made in order to investigate the interaction of radiotherapy and cytostatic drugs in the sound kidney. The authors have submitted 240 white female Wistar rats to ablation of the right kidney and to extracorporal irradiation of the left kidney with increasing doses of 10 to 50 Gy. One half of these animals received additionally an intravenous injection of 6 mg adriamycin per kg of body weight. Another group of 24 animals was only treated with adriamycin, 24 animals were only operated, and 24 animals were not treated and served as control group. The laboratory examinations of blood and urine as well as the light-microscopic investigation of the treated kidneys showed already after 10 Gy modifications of the renal parenchyma which have to be imputed to irradiation. A threshold dose cannot be deducted from this assay. All morphologically demonstrable cell lesions were progressive. Adriamycin had a nephrotoxic influence on the parenchyma of the sound kidney which showed a compensatory hypertrophy. Combined treatment with ionizing radiation and adriamycin of the sound kidney of the rat showing a compensatory hypertrophy had a synergistic effect on the renal parenchyma and leaded to progressive nephropathy.