Catecholamines have been implicated in neuromodulation of peripheral chemosensitivity and central respiratory mechanisms. Because glucocorticoids can affect catecholamine metabolism in the carotid body and brainstem, this study explored the possibility that, in rats, dexamethasone or adrenalectomy affects catecholamine biosynthesis in carotid body chemoreceptors and the medullary areas (A2C2, A5, A6, A7) involved in the chemoreflex pathway and the hypoxic ventilatory response (HVR). One dexamethasone injection (1 mg/kg body wt.) stimulated tyrosine hydroxylase activity in the carotid body and had no effect in brainstem catecholamine areas, while HVR was reduced. Chronic dexamethasone (1 mg/kg body wt. daily for 10 days) had a stimulatory influence on tyrosine hydroxylase activity in the carotid body and an inhibitory effect on A2C2, A5 and A7 cell groups. Breathing pattern, but not HVR, was altered. Adrenalectomy elicited an increase in tyrosine hydroxylase activity in A2C2, which was accompanied by a decreased respiratory frequency in hypoxia. The data show that glucocorticoids have differential effects on catecholamine biosynthesis in peripheral and central structures involved in the chemoreflex pathway. Depending on the treatment, the neurochemical changes were accompanied by alterations of HVR or the breathing pattern, which are consistent with a neuromodulating influence of catecholamines on peripheral chemosensory inputs or the central respiratory network.