A pituitary tumor was induced in a female inbred BUF rat on an 18-month diet containing 1.1 mmole 2,4,6-trimethylaniline/kg. In the sixth transfer, this tumor developed into two lines of transplantable tumors with different characteristics. Here these two lines were studied by light and electron microscopy; histologically, both tumors were well-differentiated pituitary carcinomas. In the ultrathin sections, the neoplastic cells were separated by a wide intercellular space and covered by numerous microvilli. In the mammosomatotrophic tumor (7315a) the neoplastic cells contained big, electron-dense, ovoid or globular secretory granules (560-1,700 nm in size) that were similar to the prolactin granules of mammotrophs. However, in these cells were also small, pale, uniform, secretory granules that were along the plasma membrane, measured 160 nm in diameter, and resembled the ACTH-containing granules of corticotrophs. The neoplastic cells in tumor line 7315i possessed secretory granules comparable to the granules of the ACTH-secreting cells. The differentiation of the ergastoplasm was abnormal. The tumor cells contained an endoplasmic reticulum similar to mammotrophs and somatotrophs but dissimilar to ACTH-secreting cells. These investigations suggested that the production of several hormones in transplantable pituitary tumors resulted from the multisecretory differentiation of one neoplastic pituitary cell.