The aim of this study was to investigate the participation of the 26S proteasome in the regulation of progesterone receptor (PR) concentrations in the rat brain in vivo. Ovariectomized adult female rats were treated with estradiol (10 µg/100 g s.c.), estradiol + progesterone (400 µg/100 g), and vehicle (corn oil/10% ethanol) in the presence or absence of the proteasome inhibitor Z-Ile-Glu (OBu<sup>1</sup>)-Ala-Leu-H (PSI, 300 µg/100 g). Proteins were extracted from the preoptic area, the hippocampus, and the frontal cortex, and processed for Western blot. Estradiol-induced PR expression in the preoptic area and the hippocampus, whereas progesterone did not modify the effect of estradiol. Neither estradiol nor progesterone modified PR content in the frontal cortex. PSI treatment increased PR content in the preoptic area and the hippocampus. This increase was significant in both regions after 24 h of the treatment with progesterone + PSI in the animals primed with estradiol. In this case, the content of both PR isoforms (PR-A and PR-B) was increased in a similar manner by PSI in the preoptic area (90 and 97%) and in the hippocampus (49 and 50%). PSI did not affect PR content in the frontal cortex. Our results suggest that the 26S proteasome could participate in the turnover of PR in the preoptic area and the hippocampus of the rat in vivo.