During investigations of the nuclear binding sites (acceptor sites) for the progesterone-receptor (P-R) in the developed chick oviduct, significant modulations in the capacity of various preparations of the receptor to bind to these nuclear acceptor sites were observed. These modulations occurred with a seasonal periodicity based on the date of the receptor isolation, demonstrating a nonfunctional P-R in the winter and a functional P-R during the rest of the year. Seasonal rhythms were observed for the nuclear binding of P-R to oviduct nuclei both in vivo and in vitro and to partially purified nuclear acceptor sites composed of protein-DNA complexes termed nucleoacidic protein (NAP). The rhythms were accompanied by a rhythm in the steroid-induced alterations of RNA polymerase II activity. No rhythms were apparent using pure DNA in the cell free binding assays, suggesting that pure DNA alone does not represent the native acceptor sites. These modulations in nuclear binding were found to correlate with changes in one of the two species (the A species) of the P-R. No other physico-chemical parameter (e.g., affinity of the steroid for the receptor, changes in sedimentation patterns, etc.) was identified which distinguished the receptor with nuclear binding capacity from the receptor with no such activity. Nonfunctioning P-R has also been identified in the undeveloped oviducts of untreated chicks and the atrophied oviducts of estrogen withdrawn chicks. In these instances, the B species of the P-R is missing. These results suggest that the two species of P-R from the avian oviduct are required for either activation or for the translocation and binding of the P-R in the nucleus. Whether the loss in these species is a cause or a result of receptor inactivation remains to be determined.