Recordings of 10 rats were obtained during the first 8 h of the light period under control conditions and after 24 h of sleep deprivation (SD). Non-rapid eye movement sleep (non-REMS) and EEG spectra in the range of 0.5-4.0 Hz were analyzed for 4 sec epochs. The time course of EEG slow wave activity within non-REMS episodes was closely approximated by a saturating exponential function. The time constant of approx. 50 sec varied little over consecutive 2 h periods and was not significantly changed by SD. The maximal level of EEG slow wave activity attained within non-REMS episodes (corresponding to the asymptote level of the fitted function) decreased over consecutive 2 h periods and was massively enhanced after SD. The study documents both the invariant and the homeostatically regulated aspect of the intraepisodic build-up of EEG slow wave activity.