Specific binding of estradiol was measured in both cytoplasmic and nuclear fractions of uterine arteries obtained from ten women (39–52 years of age) undergoing hysterectomy. Seven patients were menstruating regularly and three were perimenopausal. Whereas nuclear receptors could be quantified in all ten tissues, cytoplasmic receptors were detected only in four. The mean receptor concentrations in the nuclear and cytosolic fractions were similar (31–42 fmol/mg protein), and the apparent dissociation constant of the receptor from all tissues was 1–2 nM. There was no relationship between the age and nuclear receptor concentration, but there was a tendency towards a negative correlation between the plasma progesterone:estradiol ratio and receptor levels. The presence of estrogen receptor in the nuclear fraction and its absence in the cytoplasm may warrant modification of the classical two-step model for steroid hormone action.