The effects of prolactin on various tissues have been reviewed recently by Sulman (1970). In the search for target receptors of prolactin, the capacities of pigeon crop sac mucosa and of the rat mammary gland to bind labelled prolactin were investigated. Cox (1951) used labelled prolactin for similar studies in mice and found appreciable amounts of radioactivity in the mammary glands but noticed rapid breakdown of the labelled hormone. For the present study two methods were used: (a) radioactivity was measured in mammary glands of lactating rat mothers 1–12 h after i.v. injection of 125I-labelled prolactin; (b) mucosal proliferation and uptake of radioactivity by the pigeon crop were measured after local injection of 125I-labelled prolactin, 125I-labelled human chorionic gonadotrophin (HCG), 131I-labelled albumin or Na125I. Iodination was carried out according to Greenwood, Hunter & Glover (1963), using Na125I (Amersham) and PS-9 prolactin (N.I.H.)