In rabbit fetal placenta, 17β-estradiol and progesterone are not bound by specific (high affinity, low capacity) components in the cytosol, in spite of the fact that this part of the organ is very reactive to modifications in hormonal conditions. Cytosol receptors for estradiol and progesterone are found only in the maternal part of the placenta; apparent association constants are about 3 × 109M-1 for 17β-estradiol and about 2 × 108M-1 for progesterone (in the absence of glycerol) at 4 °C. Estradiol is also bound to nuclei in cells of the maternal placenta in vivo and in vitro. Concentrations of estradiol and progesterone binding sites show marked variations during normal gestation in the rabbit. For estradiol the number of binding sites increases from day 10 to day 14, then decreases until parturition. This number increases for progesterone from day 10 to day 16, then decreases until day 27.