Small time differences between the EEG activities of two channels were measured by a method based on the use of coherence and phase spectra over a certain frequency range. In many cases this method allowed to establish that time differences of the order of 5-50 msec were actually present between two channels which appeared synchronous on visual inspection. The method was applied to seizure activity from the penicillin and kindling models in the cat and to seizures recorded from scalp and intracerebral electrodes in epileptic patients. When the seizure activity was widespread but was known to be related to an epileptic focus, it was found that the area of the focus had a consistent time lead over the other recording sites. It was concluded that the method could frequently allow to: (1) assess the presence of an epileptic focus even when only widespread seizure activity could be recorded; (2) make inferences about the possible routes of propagation of seizure activity.