Melatonin attenuates carotid chemoreceptor response to hypercapnic acidosis and may contribute to the effect of circadian rhythms on the chemoreflex. The purpose of this study was to test the hypothesis that melatonin modulates rat carotid chemoreceptor response to hypoxia. To examine the effect of melatonin on the hypoxic response of the chemosensitive cells, cytosolic calcium ([Ca2+]i) was measured by spectrofluorometry in fura-2-loaded type-I (glomus) cells dissociated from rat carotid bodies. Melatonin (0.01-10 nm) did not change the resting Ca2+]i level of the glomus cells but it concentration-dependently increased peak Ca2+]i response to cyanide or deoxygenated buffer. An agonist of melatonin receptors, iodomelatonin also enhanced the Ca2+]i response to hypoxia. The melatonin-induced enhancement of the Ca2+]i response was abolished by pretreatment with nonselective mt1/MT2 antagonist, luzindole, and by MT2 antagonists, 4-phenyl-2-propionamidotetraline or DH97. These findings suggest that melatonin receptors in the glomus cells mediate the effect of melatonin on the chemoreceptor response to hypoxia. In addition, melatonin increased the carotid afferent response to hypoxia in unitary activities recorded from the sinus nerve in isolated carotid bodies superfused with bicarbonate-buffer saline. Furthermore, plethysmographic measurement of ventilatory activities in unanesthetized rats revealed that melatonin (1 mg/kg, i.p.) increased the ventilatory response to hypoxia. Hence, the circadian rhythm of melatonin in arterial blood can modulate the carotid chemoreceptor response to hypoxia. This modulation may be a physiological mechanism involved in the day-light differences in ventilatory activities.