In the adrenal medulla of adult rat, physiological levels of glucocorticoid hormones are required to maintain the catalytic activity of the epinephrine-synthesizing enzyme, phenylethanolamine N-methyltransferase (PNMT). The present study was undertaken to determine whether glucocorticoid regulation of PNMT occurs at the level of mRNA coding for PNMT. Adult male Sprague-Dawley rats were hypophysectomized (HPX) and killed after 2 weeks; pellets of corticosterone were implanted for 1, 3 or 7 days prior to killing. Determinations were made of plasma corticosterone levels, adrenal PNMT activity and PNMT mRNA levels by Northern gel analysis. HPX resulted in a decrease in plasma corticosterone to undetectable levels, and decreases in PNMT activity and PNMT mRNA levels to 1 and 18% of the levels observed in sham rats, respectively. Corticosterone replacement produced high prolonged plasma levels of corticosterone which were 10 times those of sham rats, and significantly increased levels of PNMT activity and mRNA. However, corticosterone replacement failed to restore PNMT activity and mRNA levels fully. These results suggest that the maintenance of PNMT mRNA levels is dependent on maintaining corticosterone levels and supports the hypothesis that PNMT gene expression in the adrenal medulla is directly regulated by glucocorticoids produced by the adrenal cortex. However, the results also suggest that in the chronically HPX rat, factors in addition to naturally produced glucocorticoids are required for full restoration of PNMT mRNA levels.