Rapid eye movement (REM) sleep deprivation and auditory stimulation (ADS), separately, increase REM sleep in rats, cats and humans. The main goal of the present study was to test whether administration of ADS during REM sleep rebound has a synergistic effect on REM sleep elicitation. Male Wistar rats were implanted with standard sleep recording electrodes. Following the recovery period, animals were randomly assigned to the following conditions: undeprived (i.e. control) and 24, 48, 96 and 120 hours of REM sleep deprivation by the platform method. Undeprived and REM sleep-deprived animals were divided into two groups, with and without ADS. ADS was a "beep" of 80 dB and 2,000 Hz, lasting 20 msec every 10 seconds. This stimulus was applied for the first 4 hours of sleep recordings after deprivation. After that, animals were recorded for another 4 hours. In the undeprived situation, the group that received ADS increased REM sleep approximately 70% above the group that did not receive ADS, as has been reported previously (REM sleep without ADS: 38.1 +/- 13.84 vs. with ADS: 64.6 +/- 11.8, p < 0.005). No synergistic effect was observed between REM sleep deprivation and ADS for any REM sleep-deprivation schedule. This result may be explained as an increase in the excitability pattern of pontine neurons and/or changes in the cholinergic system due to REM sleep deprivation that could not be further increased by ADS.