Pituitary tumors in rats can be induced by estrogens. They also occur spontaneously in various frequencies depending on strain, sex and age of the animals. They were considered to be hormone-dependent (mammotropic). However, morphologically they were mostly described as chromophobe pitutary adenomas. This does not contradict hormonal activity if one assumes that a degranulation during this activity in tumor cells takes place. METHODS: 81 pituitary adenomas were analysed histologically. They had been observed during experiments with resorptive carcinogens which either produced tumors in the intracranial cavity or were supposed to do so. One tumor was transplanted subcutaneously. Several passages of this tumor were followed up histologically and by electron-microscopy. In addition, these tumors were explanted in vitro. RESULTS: Pituitary adenomas were found in rats which were 623 ± 200 days old; 74 were females, 5 were male. The morphology of rat pituitary tumors was characterized by two cells types: the one consisted of small dark cells that were considered to be stromal elements; the other cells, the true adenoma cells, were epithelial elements in which no specific granulations could be found with the usual methods. Only few tumors had mitoses, infiltrative growth and celular pleomorphism. Tumors were rich in vasculature. Transplanted tumors grew regularly in isogenic rats. The induction time was over 200 days in the first six and about 150 days in the 7 th-10 th passages. Histologically, these tumors were not different from the primary adenomas. In parts, the stromal participation was more pronounced. By electron microscopy highly differentiated cells could be identified. They had many mitochondria, endoplasmic reticulum, vesicles and dispersed hormone granula. Some cells had a very high number of mitochondria, thus resembling oncocytes. The explantation of transplant pituitary adenomas was not successful. Fibroblasts always overgrew the adenoma cells, which had apparently a very low proliferation rate. These tumors probably are hormone-dependent and hormonally active. They can be considered to be spontaneous tumors in rats. However, they exhibited autonomous growth potentialities when transplanted into other hosts. These are benign tumors which have possibility of malignant growth.