The development of estrogen-induced pituitary prolactinoma in Fischer 344 (F344) rats is associated with enhanced neovascularization. This type of tumor is a rich source of basic fibroblast growth factor (bFGF), which possesses strong mitogenic and angiogenic properties. Pentosan polysulfate sodium (PPS) has been shown to exert antitumor activity by antagonizing the binding of bFGF to cell surface receptors. We have examined the effects of pentosan on tumor growth, hyperprolactinemia and angiogenesis in diethylstilbestrol-induced anterior pituitary adenoma in F344 rats. Chronic treatment with PPS did not cause any changes in the pituitary weight and serum prolactin concentration in comparison with untreated animals. The density of microvessels identified by CD-31 was also not affected by the tested drug. On the other hand, pentosan has been found to decrease cell proliferation evaluated by a number of PCNA-positive stained cell nuclei. Moreover, the TUNEL method has revealed an increased number of apoptotic bodies within the anterior pituitary after treatment with PPS. Despite the antiproliferative and proapoptotic activity of pentosan, the drug failed to inhibit tumor growth. This fact might be due to the lack of antiangiogenic activity of PPS in this experimental design.