In target tissues for estrogen, including breast cancer cells, the synthesis of progesterone receptors (PRs) is controlled by estradiol acting through estrogen receptors (ERs). We describe studies with T47D human breast cancer cells, whose PRs are not regulated by estradiol, though present in extraordinary amounts (300,000 sites per cell). These cells have no ERs sedimenting at 8S on sucrose density gradients, and no unfilled cytoplasmic or nuclear ERs; some apparently hormone-filled nuclear sites, with KD congruent to 0.7 nM, can be demonstrated by exchange. The nuclear ER sites are not processed after estradiol treatment. Nafoxidine, however, doubles nuclear estrogen binding in 6 hr, in a cycloheximide-insensitive step that may represent a reversal of processing. T47D cells are profoundly resistant to estrogens and antiestrogens; estradiol does not stimulate PRs, and nafoxidine concentrations that are cytotoxic to ER-positive cells have no effect on cell growth or on PR levels. Yet the PRs are normal by several criteria, and they can be stoichiometrically translocated to, and extracted from, nuclei in the first 3 min after progesterone addition. If progesterone treatment exceeds 10 min, rapid nuclear turnover prevents quantitative PR recovery. Cytoplasmic PRs are replenished in 10 to 24 hr, and this cycloheximide-sensitive step is also estrogen- and nafoxidine-resistant. However, despite their insensitivity to estradiol or antiestrogen, PRs are not constitutively synthesized; 5-bromodeoxyuridine and sodium butyrate can selectively inhibit PR production. Thus, since PRs retain some characteristics of inducible proteins, the persistent nuclear estrogen-binding sites may be stimulating PRs continuously, even in the absence of exogenous estradiol.