Renal lesions developed in Osborne-Mendel male and female rats ingesting dieldrin or aldrin in the diet. Chronic interstitial nephritis was seen in rats surviving for 52 wk or longer. The incidence of nephritis was highest and the lesion was most severe in male rats given the higher dose levels of dieldrin, 50 ppm or higher. Over one-half of the rats fed dieldrin or aldrin at 150 ppm, and many fed 100 ppm, died from renal necrosis and sometimes hepatic necrosis during the first year. More female rats died from renal necrosis than did male rats. Rats dying from renal necrosis did not develop tumors; those from severe chronic nephritis either did not have tumors or had preneoplastic lesions that would have become tumors if the animal had lived longer. Thus acute and chronic effects should both be examined carefully when evaluating the safety of a chemical. In addition to causing the death of the animal, acute and chronic toxic effects can prevent the development of malignant tumors by shortening the animal's life span or by causing illness and inhibiting the development of a tumor that otherwise might occur in a healthy animal.