High-threshold primary afferent units were selected from the population of axons functionally isolated in fine filaments dissected from the median and ulnar nerves of macaque monkeys. In 47 experiments, 536 such units were studied. They comprised 345 A-delta and 191 C fiber units; 90 low-threshold mechanoreceptive afferents were also studied for comparison. The high-threshold units were categorized by observing their responses to mechanical and thermal (53 and 8 degrees C) stimuli applied to their receptive fiblds in the glabrous skin of the hand.