Ovariectomy and estrogen (E) or E plus progesterone treatment has previously been shown to alter both hypothalamic content and portal plasma levels of beta-endorphin. To determine if these changes were accompanied by changes in beta-endorphin synthesis, we used a RNA dot blot method to quantify proopiomelanocortin (POMC) mRNA levels in the arcuate nucleus-median eminence region of rats. Animals were bilaterally ovariectomized, implanted with Silastic capsules containing E or oil, and killed 1 or 3 days after implantation. Total nucleic acid was isolated from dissections of the arcuate-median eminence by proteinase-K/sodium dodecyl sulfate/phenol extraction, and POMC mRNA was quantified by dot blot analysis. Although 1 day of E treatment had no effect on hypothalamic POMC mRNA levels, 3 days of E treatment caused a significant reduction of approximately 40% of POMC mRNA levels relative to oil controls in two replicate experiments. These results suggest that the decreases in hypothalamic POMC peptide levels after E administration reported previously may be due to a decrease in POMC peptide biosynthesis resulting from a decrease in hypothalamic POMC mRNA.