Melatonin modulates the carotid chemoreceptor response to chemical stimuli, and chronic hypoxia changes circadian activities and carotid body function. The purpose of this study was to test the hypothesis that chronic hypoxia alters the function and expression of melatonin receptors in the rat carotid body. Effects of melatonin on the carotid responses to hypercapnic acidosis and to hypoxia were determined by spectrofluorometric measurement of cytosolic calcium ([Ca(2+)](i)) in fura-2-loaded type-I (glomus) cells dissociated from carotid bodies obtained from normoxic (Nx) or chronically hypoxic (CH) rats breathing 10% oxygen for 4 wk. In the Nx control, melatonin concentration dependently attenuated the peak [Ca(2+)](i) response to hypercapnic acidosis, whereas it augmented the [Ca(2+)](i) response to cyanide or deoxygenated buffer. Yet, melatonin enhanced the peak [Ca(2+)](i) responses to hypercapnic acidosis or hypoxia in the CH glomus cells. An agonist of melatonin receptors, iodomelatonin also elevated the hypercapnic or hypoxic responses in the CH groups. The melatonin-induced changes in the [Ca(2+)](i) responses were abolished by pretreatment with nonselective mt(1)/MT(2) antagonist, luzindole, and by MT(2) antagonists, 4-phenyl-2-propionamidotetraline or DH97. These findings suggest a functional modulation of melatonin receptors in the glomus cells in chronic hypoxia. To evaluate the level of expression of the melatonin receptors, in situ hybridization study with antisense mt(1) and MT(2) receptor mRNA oligonucleotide probes was performed on the Nx and CH carotid bodies. There were significant increases in the expression of mt(1) and MT(2) receptors in the CH comparing with the Nx group. Taken together, our results suggest an upregulation of the carotid expression of melatonin receptors by chronic hypoxia, which modulates the carotid response to melatonin for the circadian influence on breathing.