A spontaneous mammary adenocarcinoma was detected in a 10-wk-old female virgin Sprague-Dawley rat in a subacute toxicity study for safety assessment. The rat was sacrificed according to the study schedule at 23 wk of age and was subjected to complete necropsy. Gross pathological examination revealed that the tumor had reached 20 x 35 x 45 mm. It was settled in the right posterior abdominal subcutaneous tissue and had not invaded the surrounding tissues. Histopathologically, it was diagnosed as a typical mammary adenocarcinoma of rats. In our literature search, we found no previous reports of spontaneous adenocarcinomas of the mammary gland in such a young rat. Therefore, attention should be paid to the fact that mammary adenocarcinomas can occur spontaneously at an early age in rats.