EEG was continuously recorded in 15 patients for a period extending from just before to 1/2 hour after unilateral ECT. Fourier analysis was performed on the EEG following 15 right-sided treatments and five left-sided treatments. During the induced seizure, epileptic slow-wave activity had significantly greater power on the treated side. Immediately after the seizure, there was significantly more delta activity and less alpha and beta activity on the treated side. This asymmetry, though becoming less marked, was usually still present at the end of the recording period. Analysis of other variables associated with the treatment showed that there was a significant correlation between the time to eye-opening after ECT and both the duration of the seizure and the amount of anaesthetic administered. The similarity between these induced unilateral seizures and unilateral seizures occurring spontaneously in some epileptics is discussed.