Prolactin is present in the reptilian pituitary (Grignon & Herlant, 1959; Nicoll & Bern, 1965; Nicoll, Bern & Brown, 1966) and its cellular site of origin has been ascribed to the caudal acidophils (Licht & Nicoll, 1969). This investigation is part of a study of the role of prolactin in the reproductive processes of female lizards. Dipsosaurus dorsalis obtained during June-September were housed in wire mesh cages (60 × 30 × 30 cm., 10/cage) for 14 days with an available temperature gradient of 21–46° and a photoperiod of 16 hr. light-8 hr. dark. Food consisted of lettuce and water ad libitum. Growth hormone (GH), 200 μg. NIH-GH-S9 (0·2 i.u.)/100 g. body wt/day, and ovine prolactin, 1·0 mg. NIH-P-S8 (25 i.u.)/100 g. body wt/day, were administered s.c. in alkaline saline, and pregnant mare serum gonadotrophin (PMS, Ayerst 'Equinex '), 25 i.u./day i.p. in saline. Hypophysectomy by a buccal approach was verified by