In several previous studies nuclear accumulation of the progesterone receptor was significantly lower than the quantity depleted from the cytosol one h after progesterone injection. The results presented herein indicate that this apparent lack of stoichiometry is due to loss of detectable receptor from the nucleus during the nuclear washes and after the assay incubation. This decrease in measured receptor results both from solubilization of the receptor-progesterone complex into the supernatant and from dissociation of [3H] ligand from the receptor. Conversely, no significant quantities of receptor were detected in the mitochondrial/microsomal fraction, preincubation nuclear washes, second ethanol extraction of the nuclear pellet, and soluene digest of the extracted pellet. Thin layer chromatography of the radioactive ligand bound to the nuclear receptor after in vitro exchange confirmed that virtually all the specific binding was due to [3H] progesterone, in spite of a 30% conversion to other metabolites in the incubation fluid.