A spontaneous tumor was found in the left adrenal gland of a 35-day-old male Wistar rat. Histologically, the tumor consisted of various tissues derived from all three embryonic germ layers, i.e. the ectoderm (nervous tissue and skin), mesoderm (striated muscle, bone and cartilage), and endoderm (glandular tissue). The tumor also contained loosely or compactly arranged embryonic connective tissues. From these findings, the tumor was diagnosed as teratoma. This appears to be the first description of teratoma in the adrenal gland of the rat.