In the present study we used the model of cultured somatostatin-receptor-positive prolactin (PRL)-secreting rat pituitary tumor cells to investigate the possible interrelationships between the anti-mitotic and hormone-release-inhibitory effects of the somatostatin analog octreotide (SMS 201-955) and the effects of vincristine, methotrexate, fluouracil and suramin. Dose-dependent inhibitory effects of all compounds were shown both on the DNA content and on PRL release. Octreotide and these cytostatic compounds were slightly additive in their anti-proliferative and anti-secretory effects. The somatostatin analog did not alter drug sensitivity in these tumor cells, however. The data obtained in this tumor model suggest that octreotide can be effectively administered in combination with cytostatic drugs and/or suramin.