Ten of 16 patients with complex partial epilepsy displayed an interictal spike-slow wave sequence with characteristic morphology and depth voltage topography. This 'typical slow wave' (TSW) lasted 300-600 msec, was usually largest and negative in the anterior hippocampus, and positive in the amygdala. Simultaneous recordings from ipsilateral cingulate, supplementary motor, orbitofrontal, and lateral temporal cortices, as well as from the contralateral medial temporal lobe (MTL), revealed only small, apparently volume-conducted, wave forms. Simultaneously recorded multiunit activity within the focal MTL was profoundly inhibited during the TSW. The TSW propagated to the scalp, producing a large widespread positivity. A large endogenous potential with similar latency range and task correlates as the scalp-P3 was recorded from the MTL to infrequent tones in a simple discrimination task. This 'depth-P3' had very similar polarity and relative amplitude across MTL sites, as was observed for the TSW at the same electrode contacts. However, at more superficial intracranial sites, the TSW was relatively smaller than the P3. Similarly, from MTL to surface, the P3 was found to decrement about half as much as the TSW decrements. This evidence suggests that the surface P3 is generated, but in part only, by the MTL.