The dynamics of EEG slow wave activity during sleep were investigated in two subjects recorded for 14 consecutive nights, and in 14 subjects recorded for one night after placebo administration. In addition, records were obtained after a single bedtime dose of the benzodiazepine hypnotics flurazepam (30 mg), flunitrazepam (2 mg), triazolam (0.5 mg) and midazolam (15 mg). Mean slow wave activity (i.e. spectral power density in the 0.75-4.5 Hz band) invariably declined from the first to the third nonREM sleep episode. Within episodes, slow wave activity showed initially a gradual buildup over a period of offroximately 35 min, and in the end a rapid decline. Both the rise rates and fall rates decreased over the first three nonREM sleep episodes. The benzodiazepines typically attenuated mean slow wave activity within episodes as well as the rise and fall rates. For three compounds, residual effects were demonstrated in the drug-free post-drug night. We conclude that a homeostatically regulated sleep process determines the buildup rate of slow wave activity within nonREM sleep episodes.