This report demonstrates that the chick oviduct, a specific target organ for progesterone, contains both cytoplasmic and nuclear macromolecules which bind progestins. These binding molecules can be clearly distinguished from transcortin by centrifugation through sucrose gradients of low ionic strength and by agarose gel filtration. The cytoplasmic progesterone-binding molecules also bind 5-alpha-pregnane-3,20-dione, but have significantly lower affinity for cortisol, estrone, or aldosterone. They are absent from blood and nontarget organs such as lung and spleen. The tissue-specific binding components appear to be heat-labile proteins with an average dissociation constant for progesterone of about 8 x 10(-10) M at 2 degrees C. These results are consistent with the identification of the progesterone-binding molecules as the functional hormone receptors. In further support of this concept is the finding that treatment of the chicks with estrogen coordinately induces a 20-fold increase in the number of progesterone-binding molecules and enhances the capacity of progesterone to induce avidin synthesis.A progesterone-"receptor" complex can be detected in both the cytoplasm and nuclei of oviduct tissue after an injection of [(3)H]progesterone to estrogen-treated chicks. By contrast, incubation of oviduct tissue with [(3)H]progesterone in vitro at 2 degrees C for 5 min leads to labeling of the cytoplasmic "receptor" only. Transfer of the "receptor"-steroid complex into the nucleus then appears to occur upon subsequent incubation in vitro at 37 degrees C. This observation suggests that the transfer of bound progesterone across the nuclear membrane may be an energy-requiring enzymatic process.