Adenomatous cells obtained from a pituitary tumor induced in Fisher 344/Lis rats by the subcutaneous implantation of estrone (E<sub>1</sub>) were found to secrete large amounts of prolactin (PRL). The secretion of PRL was stimulated by thyrotropin-releasing hormone (TRH) and low concentrations of dopamine (DA), while micromolar concentrations of DA were inhibitory. High affinity binding sites for <sup>3</sup>H-spiroperidol (<sup>3</sup>H-SPIR) were found to be present on the cells and to conform to the criteria of dopaminergic receptors. An adenylate cyclase (AC) present in the cells could be activated by a guanyl nucleotide and was inhibited by DA in the presence of guanosine 5´-triphosphate (GTP). Fractionation of the adenomatous cells by Percoll gradients identified two groups of cells capable of secreting PRL and bearing <sup>3</sup>H-SPIR binding sites. These data indicate that this rat pituitary adenoma may be a model for human prolactinomas that might be utilized for the study of the mechanism of action of dopaminergic drugs.