Pituitary and plasma prolactin levels have been measured in the golden hamster (Mesocricetus auratus) by means of a double antibody radioimmunoassay (RIA) using the NIAMD rat prolactin kit. The variance between the rat prolactin standard curve and the hamster pituitary dilution curve meant that all experimental data in the hamster had to be compared to a preparation of standard hamster anterior pituitary tissue (SHAP). Pituitary and plasma SHAP values in the hamster have been shown to fluctuate in a manner completely analogous to reported prolactin changes in other mammals, under the following conditions: stress, lactation, inhibition of pituitary release in vitro by hypothalamic extract, and pineal activation. It is concluded that this system can accurately estimate prolactin levels in the hamster in both pituitary and plasma samples.