Reserpine treatment has been shown to cause a long-lasting decrease in phenylethanolamine N-methyltransferase mRNA levels and a simultaneous increase in tyrosine hydroxylase and neuropeptide tyrosine mRNA levels in chromaffin cells of rat adrenal medulla. In this study, in situ hybridization histochemistry was used to further investigate factors involved in the differential regulation of the catecholamine synthesizing enzymes and the coexisting peptide neuropeptide tyrosine. Pretreatment with the synthetic glucocorticoid analogue dexamethasone followed by the administration of a single dose of reserpine completely reversed the decrease in phenylethanolamine N-methyltransferase mRNA seen after reserpine treatment alone, but had no effect on the reserpine-induced increase in tyrosine hydroxylase mRNA. Dexamethasone alone did not change phenylethanolamine N-methyltransferase or tyrosine hydroxylase mRNA levels in the adrenal medulla. When reserpine-treated rats were given adrenocorticotropic hormone a partial reversal of the decrease in phenylethanolamine N-methyltransferase mRNA was seen. Furthermore, the reserpine-induced increase in neuropeptide tyrosine mRNA levels was markedly reduced when animals were pretreated with dexamethasone, whereas dexamethasone alone had no effect on neuropeptide tyrosine mRNA levels. The drop in phenylethanolamine N-methyltransferase mRNA levels after reserpine treatment was not due to a depression of the pituitary adrenal axis, since proopiomelanocortin mRNA levels in the anterior pituitary increased and plasma corticosterone levels were stable following reserpine treatment. A possible local regulation within the adrenal gland that may involve the glucocorticoid receptor and/or other factors is discussed.