GH3 cells are shown to contain cytosolic progesterone receptor in a mean concentration of 270 fmol/mg protein with a dissociation constant (K<sub>D</sub>) for promegestone of 3 × 10<sup>-9</sup> M (4°C). Estrogen receptor (K<sub>D</sub> = 1.8 × 10<sup>-10</sup> M) is demonstrated in cytosol (121 fmol/mg protein) as well as in 0.4 M KCl extracts (89 fmol/mg protein) of crude nuclear fractions. No progesterone receptor was detectable in the nuclear fraction. Both receptors are characterized by isoelectric focussing and by competition experiments. Addition of 10<sup>-8</sup> M estradiol to the growth medium increases progesterone receptor levels up to five, suggesting that the progesterone receptor is under estrogenic control.