Steroids enter target cells and bind to specific receptor proteins. These complexes translocate to the nuclei, bind to the chromatin, and alter gene expression. Recently, inactive progesterone receptors of the chick oviduct have been identified in this laboratory during the late winter which are not capable of translocating and binding to nuclear acceptor sites either in vivo or in a cell free assay. During this period the oviduct remains unresponsive to the steroid. The nuclear binding activity does return at the end of the season with a corresponding return of oviduct responsiveness to the steroid. Analysis of the active and inactive receptors of progesterone reveals no difference in sedimentation rates in high salt or in the affinity of the steroid for the receptor. The tissue levels of the inactive receptor, however, are about-one-half those of the active receptor. Quantitative analysis of the molecular species of the progesterone receptor separated by isoelectric focusing reveals two species for the active receptor preparations (an A species focusing at a pH of 7 and a B species focusing at a pH of 6). The A species was absent in the active receptor preparations which explains the lower amounts of total receptor in this group. Further studies have shown inactive progesterone receptors in the undeveloped oviducts and in the oviducts of estrogen withdrawn chicks. In these instances, the B species of the receptor is missing. The results suggest: (1) a novel regulation of steroid action may exist which acts by modulating the levels of one of the two receptor species (or possible subunits of a dimer); (2) the presence of a steroid receptor does not necessarily reflect that the receptor is functional; and (3) the biological activity of a steroid may be assessed via cell free nuclear binding assays or via analysis of the molecular species.