Lesions in either the anterior cingulate cortex (ACC) or the caudate nucleus (CN) impair avoidance behavior from noxious somatic stimuli, so these two areas may play a similar role in pain-avoidance behavior. To test this hypothesis, we recorded single neuronal activities in the ACC and in the CN of a monkey while it was performing a pain-avoidance task. Ten of 136 ACC and eleven of 160 CN neurons responded selectively during pain-avoidance behavior. We found little difference in the population distribution or in the response latency and duration. Our present findings are in accordance with previous lesion and anatomical studies which suggest that these two regions could function as one module in pain-avoidance behavior.