Cerebral electrical activity was recorded through chronic stereotactically implanted electrodes in 19 epileptic patients suffering from different types of severe and medically refractory partial seizures and who were considered for surgical treatment. 213 brain sites, in all cerebral lobes, in neocortical as well as in archicortical structures, were explored. The behaviour of the interictal spiking across wakefulness and nocturnal physiological sleep was analysed, using automatic elaboration. (i) Spike rate is affected by the occurrence of sleep and by the passage from one sleep phase to another. The degree and direction of the phenomenon differ remarkably in the various patients and, in the same patient, in the different cerebral sites explored. Generally, interictal spiking increases at the beginning of sleep, reaches its maximum during the deep non-REM phases and returns to a level slightly lower than that in wakefulness during REM. (ii) The nocturnal spike rate is hardly influenced by spike location. In most cases, however, the variations recorded during sleep are more significant in the frontal regions than elsewhere. (iii) Spike rate across wakefulness and sleep is affected by the local level of epileptogenicity: spiking variations are less in the most epileptogenic cerebral zone (identified by the origin of the seizure discharges and by the disappearance of seizures following its surgical removal) than elsewhere. The physio-pathological meaning and the diagnostic value of these findings, and particularly of the peculiar stability or autonomy of the electrical epileptic activity of the most epileptogenic cerebral zone, is discussed.