The EEG and EMG were recorded during 14 nights of sleep for 5 young normal adults and were analyzed automatically with a lab computer system. From the EEG one parameter was computed which is based on the joint frequency-amplitude distribution of EEG waves. The temporal sequence of parameter values displays the time course of the sleep EEG. Another parameter, which results from the automatic analysis of the EMG, represents transient EMG activity, i.e. shortlasting changes of the muscle tone. A comparison of the automatically analyzed EEG and EMG data revealed a close correspondence between both parameters. A peak of EMG transient activity was observed in virtually each ultradian sleep cycle at a well-defined turning point between the phase of EEG synchronization and the subsequent phase of EEG desynchronization. Before and after this turning point there was a gradual decrease in the rate of transient EMG activity with minimal activity immediately preceding the turning point. The results suggest that cortical synchronization during sleep is incompatible with transient activity in the muscle system while desynchronization is invariably accompanied by a high rate of transient EMG activity.