The predictions of the two-process model of sleep regulation were tested by 12-h sleep deprivation (SDEP) and 12-h cold exposure (cold, 4 degrees C) in the rat, both carried out in the 12-h dark period. The analysis was based on recordings of vigilance states, electroencephalogram (EEG) power spectra (0.25-25.0 Hz), and cortical temperature (Tcrt). During recovery from SDEP, slow-wave activity (SWA; EEG power density in the 0.75-4.0 Hz band) in non-REM sleep was higher and the number of brief awakenings (nBA) lower than in the corresponding baseline period. During recovery from cold, a moderate and delayed increase in SWA was present. During SDEP, Tcrt was above baseline, and in some intervals of recovery below baseline. In those intervals waking was also decreased. During recovery from cold, Tcrt and waking were not affected. It is concluded that 12-h SDEP causes an intensification of sleep, as indicated by the enhanced SWA and the reduced nBA, whereas 12-h cold has only marginal effects. The time course of SWA in both experiments could be accurately predicted by a computer simulation based on the assumption of the two-process model and the parameter values determined in a previous experiment.