The possible existence of pineal progesterone (P) receptors was assessed in 105,000 g supernatants of bovine pineal glands incubated with 3H-P in the presence of 10 μM cortisol. By charcoal adsorption analysis specific binding (defined as the difference between 3H-P binding in the absence and presence of 1,000-fold excess of unlabeled P) attained its maximum after 60 min at 37 °C and after 5 h at 0 °C. Binding was abolished by heating the cytosol or by pre-incubating it with trypsin. A single population of binding sites with a Kd = 1.64 nM and binding site concentration = 34 fmol/mg of cytosol protein was detectable by Scatchard analysis. In competition experiments the B50 progesterone values (nM) were: P (2), 20α-dihydroprogesterone (12), 20β-dihydroprogesterone (25), estradiol (158), testosterone (675) and corticosterone (> 1,000). After sucrose gradient centrifugation of pineal cytosol incubated with 3H-P a discrete peak of radioactivity in the 7-8 S region of the gradient was observed, and disappeared following incubation with excess unlabeled P. These results indicate that P receptors are present in the bovine pineal gland.