Serotonin enhancing drugs, including L-tryptophan and, more recently, fluoxetine and paroxetine, have been tested as pharmacologic treatments for sleep apnea syndrome. Although some patients have demonstrated reduced apnea expression after treatment with these compounds, this improvement has been restricted to nonrapid eye movement (NREM) sleep, with some patients showing no improvement. This study reports the effects of mirtazapine, an antidepressant with 5-HT(1) agonist as well as 5-HT(2) and 5-HT(3) antagonist effects, on sleep and respiration in an established animal model of central apnea. We studied nine adult male Sprague-Dawley rats chronically instrumented for sleep staging. In random order on separate days, rats were recorded after intraperitoneal injection of: (1) saline, (2) 0.1 mg/kg +/- mirtazapine (labeled as Remeron), (3) 1 mg/kg mirtazapine, or (4) 5 mg/ kg mirtazapine. With respect to saline injections, mirtazapine at all three doses reduced apnea index during NREM sleep by more than 50% (p < 0.0001) and during REM sleep by 60% (p < 0.0001) for at least 6 h. In association with this apnea suppression normalized inspiratory minute ventilation increased during all wake/sleep states (p < 0.001 for each state). The duration of NREM sleep was unaffected by any dose of mirtazapine (p = 0.42), but NREM EEG delta power was increased by more than 30% at all doses (p = 0.04), indicating improved NREM sleep consolidation after mirtazapine injection. We conclude that mirtazapine, over a 50-fold dose range, significantly reduces central apnea expression during NREM and REM sleep in the rat. The efficacy of this compound to suppress apnea in all sleep stages most probably arises from its mixed agonist/antagonist profile at serotonin receptors. The implications of these findings for the management of sleep apnea syndrome must be verified by appropriate clinical trials.