Synthetic steroid compounds STS 557 and STS 737 administered to ovariectomized rats increased prolactin secretion to a lesser extent than estradiol benzoate or estrogen J 271. When injected to estradiol benzoate treated ovariectomized rats, compounds STS 557, STS 737 and progesterone failed to change DNA synthesis in the adenohypophysis and prolactin secretion. Only compound STS 737 exerted a pronounced effect on the adenohypophyseal cells in primary cultures. It inhibited DNA synthesis and prolactin secretion. These effects were synergistic with the inhibitory action of bromocriptine (CB-154) that is of certain interest for elaborating methods of treatment of pituitary hyperplasias and tumours.