The plasma of female rats contained progesterone-binding proteins similar to those found in luteal extracts and plasma of cows and sheep. These proteins were not ovarian in origin. The progesterone-binding activity of ovarian venous and arterial plasma of Day-16-pregnant rats and arterial plasma of rats in dioestrus was resolvable by hydroxylapatite chromatography into three components. The first component was identified as corticosteroid-binding globulin because it bound corticosterone and cortisol as well as progesterone and was not adsorbed to hydroxylapatite. The second and third components were eluted from hydroxylapatite at phosphate ionic strengths of 40 mM and 150 mM and bound progesterone with 99% less affinity. Competition experiments using the second component showed that it bound progesterone specifically. The progesterone-binding components were present in arterial plasma collected from rats which had been ovariectomized 5 days previously and in animals in which circulating concentrations of progesterone had been maintained by subcutaneous injections of progesterone for 6 days after ovariectomy.