Human reaction to cold stress and hypothermia involves shivering. Another form of overt shaking, postoperative shivering, has been attributed as a thermoregulatory response to postoperative hypothermia. Analysis of the normal human shivering pattern showed a synchronized, slow amplitude modulation (six to eight cycles/min) over all muscles sampled. In addition, there was a frequency of 8 to 10 Hz associated with each low-frequency amplitude modulation. EMG signals from postoperative patients revealed none of the major patterns seen in thermal-induced shivering. Cold-induced vasodilation also was studied and found to occur simultaneously in all cold-stressed fingers regardless of size or innervation. Thermal shivering and cold-induced vasodilation are considered to be manifestations of central neural oscillators.