The purification of canine prolactin and the development of an homologous radioimmunoassay including several physiological studies in the Beagle dog are described. The assay measured immunoreactive canine prolactin with a sensitivity limit of 0-6 ng/ml. Purified canine luteinizing hormone gave no significant inhibition in the assay whereas purified canine growth hormone inhibited the binding of 125I-labelled canine prolactin to antiserum only at very high dose levels. In Beagle dogs. basal serum prolactin concentrations were in the range 1-2 ng/ml in normal male, normal female (metoestrus and anoestrus) and oophorectomized-hysterectomized female dogs. The prolactin concentration in one sample of amniotic fluid was in the same range, while in hypophysectomized male dogs no serum prolactin could be detected by our assay system. Serum prolactin concentrations tended to increase during late pregnancy and parturition, remaining high during the first 9 days of lactation. In consequence, a negative correlation was suggested between serum prolactin and serum progesterone concentrations.