The classical model for the mechanisms of action of steroids holds that unbound receptors for steroids reside exclusively in the cytoplasmic compartment and that they undergo translocation to the nucleus when bound to steroids in a process which is temperature sensitive. In the present study we looked at the localization of the estradiol receptor using autoradiography and biochemical procedures. Uteri from ovariectomized, and/or ovariectomized-adrenalectomized rats, as well as several cell lines [with estrogen receptors (ER+)] were incubated with [3H]-estradiol or [3H]-R2858 (methoxy-17-ethinylestradiol) for 5 min to 2 h at different concentrations of ligands (0.1-10 nM) at 4 degrees C. When the tissue or cells were processed for autoradiography the localization of steroid was nuclear and cytoplasmic. In contrast when the tissue or cells were processed using the usual biochemical procedures, all binding activity appeared in the cytosolic fraction. In addition, when concentrated preparations of homogenized uteri as well as several nuclear preparation from cell lines were made, free receptor could be demonstrated in the crude nuclear preparations. In the present study, data are presented which suggest that there are unbound receptors for estrogen in nuclei of the smooth muscle cells of myometrium as well as in nuclei of the several cell lines (ER+). We propose a new model for the distribution of estrogen receptors in which unbound receptors are in equilibrium, partitioned between nucleus and cytoplasm according to the free water content of these intracellular compartments.