Chick oviduct cytosol [ 3H]progesterone-receptor complex treated with 30 m m Ca 2+ at 0 °C demonstrated a twofold greater binding to isolated chick oviduct nuclei or DNA-cellulose than such complexes activated thermally (25 °C). Divalent ions such as Mg 2+ and Mn 2+ were unable to mimic the effect of Ca 2+ under identical conditions. The capacity of the Ca 2+-treated progesterone-receptor complex to bind to nuclei or DNA-cellulose reached a peak within 45 min of Ca 2+ treatment of the complex at 0 °C. This binding gradually declined as a function of incubation time and after 24 h at 0 °C no significant binding was observed. The Ca 2+- and heat-treated chick oviduct [ 3H]progesterone-receptor complex was also characterized by DEAE-cellulose and agarose gel nitration chromatography. While heat-activated receptor could be resolved into A and B subunits on DEAE-cellulose, the receptor exposed to Ca 2+ for 45 min at low temperature yielded the “A” subunit and a broad peak with poor affinity for the anion exchanger. The peak corresponding to “B” subunit was not discernible. The broad peak which eluted before the A peak was subsequently resolved by agarose gel filtration into receptor forms IV and V as described previously by Sherman et al. ( M. Sherman, S. Atienza, J. Shansky, and L. Hoffman, 1974, J. Biol. Chem., 249, 5351–5363; M. Sherman, L. Pickering, F. Rollwagen and L. Miller, 1978, Fed. Proc., 37, 167–173). Again DEAE-cellulose chromatography of the progesterone-receptor complex treated as long as 24 h at 0 °C with Ca 2+ revealed a poorly bound peak which on agarose gel filtration corresponded exclusively to form V. A correlation was apparent between an increase in form V and a gradual decrease in the binding capacity of the Ca 2+-treated steroid-receptor complex to nuclei, DNA-cellulose, or DEAE-cellulose filters. Based on these findings, I postulate that Ca 2+ has a functional role in the mechanism of progesterone action in chick oviduct. Firstly, it enhances a low temperature, time dependent binding of the progesterone-receptor complex to chick oviduct nuclear components, and subsequently promotes, by possible activation of endogenous protease(s) the cleavage of the receptor subunits.