Modulation of specific binding of prolactin to murine mammary gland as a response to treatment with various doses of estradiol benzoate (EB) was studied. Measurements of serum and pituitary levels of prolactin were also carried out simultaneously. Estradiol benzoate at a dose of 5 microgram significantly increased the binding of 125I-labelled rat prolactin (rPRL) to mammary gland concomitant with increased serum prolactin levels in ovariectomized mice. Administration of bromoergocryptine along with EB resulted in decreased serum prolactin levels as well as binding of prolactin to breast tissue. It thus appears that the influence of estradiol on binding of prolactin to mammary gland is mediated primarily via its property of enhancing serum prolactin concentration apart from its possible direct effect at the target tissue level.