The effects of diethylstilbestrol (DES) and of long-acting somatostatin analog, octreotide (SMS) on the rat anterior pituitary microvasculature have been studied by means of computer-assisted image analysis. Additionally, the effects of DES and SMS on prolactin secretion and anterior pituitary cell proliferation have been studied, as well. The vascularization was visualized using Selye's method modified by Poely et al. (1964). The prolactin serum levels were estimated by radio-immunoassay. The proliferation indices were assessed using bromodeoxyuridine incorporation assay. As expected, it was found that DES sharply increased serum prolactin levels and enhanced cell proliferation in the anterior pituitary gland. DES also induced changes in parameters of vascularization. Simultaneous treatment of rats with SMS inhibited the DES-induced elevation of prolactin levels and pituitary cell proliferation. It also suppressed some but not all DES-induced changes in the anterior pituitary vascularization. These data suggest that the angio-inhibitory activity of SMS might be involved in its anti-tumor action on pituitary adenomas, but not as a sole or principal mechanism.