Slow brain stem auditory evoked potentials (BAEP) were recorded in humans from depth electrodes in the thalamus and from movable electrodes within the IVth ventricle (the vicinity of the pons), the aqueduct of Sylvius (the midbrain), the IIIrd ventricle (the thalamus) and the lateral ventricle. The slow potentials consist of the positive wave peaking at 5-6 msec, and two large negative waves: No (8-10 msec) and Na (12-17 msec) in the nomenclature of Picton et al. (1974). The sources of these slow negative waves have been identified, on the basis of observation of amplitude increase in depth recordings and by tracing these potentials from their intracranial maximum to the scalp. Both No and Na have their origins in the midbrain, probably representing postsynaptic activities within the inferior colliculus. No comparable slow waves were recorded from the medial geniculate body. The present observations give a basis for non-invasive evaluation of patients with midbrain dysfunction.