Only a limited number of studies is available addressing chemosensory stimulation during sleep. Recent work indicates that stimulation with a nasal irritant produces an increase in arousal frequency in non-REM sleep, whereas a selective olfactory stimulant does not. The present study focused on arousal reactions in REM sleep. Five young healthy volunteers were investigated during 27 nights. Using air-dilution olfactometry CO(2) was used for nasal irritation and H(2)S was used as a specific olfactory stimulant. Both stimuli were presented at four concentrations, odorless stimuli served as control. Other than in previous studies arousal latency was used as a dependent measure. Even the strongest olfactory stimulus did not produce an increase in arousal frequency in REM sleep whereas for irritation such an increase was clearly present. Latencies of arousal responses to CO(2) shortened with increasing stimulus concentrations. Olfactory stimulation does not lead to arousal reactions. In contrast, trigeminal stimulation produces a concentration-dependent increase in arousal frequency and decrease in arousal latency across all sleep stages. The present data shows for the first time that arousals are not present during REM sleep in response to selective olfactory stimuli. However, such changes are easily evoked by irritants activating the trigeminal nerve.