In order to test predictions of the 2-process model of sleep regulation, the effects of slow wave sleep (SWS) deprivation by acoustic stimulation during the first part of the sleep period on EEG power density and sleep duration were investigated in 2 experiments. In the first experiment, 8 subjects were deprived of SWS during the first 5 h of a baseline nocturnal sleep period without awakening. Compared to the same interval of undisturbed sleep, power densities in the delta frequencies were attenuated. In the hour following SWS deprivation, power densities in the delta and theta frequencies were considerably enhanced in comparison with the same interval of undisturbed sleep. No change in sleep duration was observed. In the second experiment, 8 subjects were sleep deprived for 1 night and recovery sleep was initiated at 11 a.m. on 2 occasions. In 1 condition subjects were deprived of SWS during the first 3 h of recovery sleep. In the other condition recovery sleep was not experimentally disturbed. During undisturbed recovery sleep, power densities in the delta and theta frequencies were higher than during baseline sleep. During SWS deprivation, power densities in this frequency range were lower than during undisturbed recovery sleep. In the hour following SWS deprivation, power densities were enhanced relative to the same interval of undisturbed recovery sleep. Again, SWS deprivation did not cause an increase of sleep duration. The observed changes in EEG power density support the hypothesis that this EEG parameter reflects the homeostatic process S.(ABSTRACT TRUNCATED AT 250 WORDS)