Spontaneous neoplastic and non-neoplastic lesions in 95 male and 96 female Donryu rats which were observed up to 120 weeks of age, were examined. The incidence of spontaneous tumors was 73.7% in males and 88.5% in females. In males, the most frequent tumors were pituitary adenomas, followed by pheochromocytomas and insulinomas. In females, uterine adenocarcinomas, mammary fibroadenomas and pituitary adenomas were the most common. Other tumors with relatively high incidences in both sexes included cortical adenomas of the adrenal gland, histiocytic sarcomas of the hematopoietic organs and granular cell tumors of the brain. Various tumors were also found in many other organs and/or tissues, although their incidences were low. The organ distribution and incidences of spontaneous tumors observed in Donryu rats were different from those in other strains of rats such as the ACI, Wistar, F344 or Sprague-Dawley strains. The main non-neoplastic lesions were observed in the lung, cervical lymph nodes and kidney of both sexes. In addition, lesions were also observed in the urinary bladder, prostate and peripheral nerves (spinal nerve roots and peripheral nerves) and/or femoralis muscle of males. Histologically, the most characteristic lesion was radiculoneuropathy with degeneration of the peripheral nerves.