The distribution and generator sources of somatosensory evoked potentials (SEPs) in the thalamus and subthalamic area were studied, using a 'semi-microelectrode' during stereotaxic surgery on 34 patients with involuntary movements or intractable pain. Electrical stimulation was given to the median nerve at the wrist. Two distinct SEPs were evoked by contralateral stimulation. A high voltage (160 microV) positive SEP with a peak latency of 15.5 msec was strictly confined to the ventral part of the sensory relay nucleus (nucleus ventro-caudalis, V.c). A much lower voltage, positive-negative-positive triphasic SEP showed peak latencies of the initial positivity and the major negativity of 13.3 msec and 16.0 msec, respectively, and had maximal voltage (16 microV) in the ventralmost parts of the nucleus ventro-intermedius (V.im) and radiatio praelemniscalis (Ra.prl), and substantial potentials in the lemniscus medialis (L.m) and nucleus ventro-oralis posterior (V.o.p). The potential field of the triphasic SEP spread farther across the different thalamic nuclei and subthalamic region with identical configurations and peak latencies, but with decreasing amplitude. These findings suggest that the high voltage positive SEP reflects a postsynaptic potential generated by the V.c neurons, and the smaller triphasic SEP a presynaptic axonal potential generated in the rostral part of the lemniscal pathway, extending by means of volume conduction.