Spontaneous post-anesthetic tremor that resembles shivering is common during recovery from anesthesia. Risks to postoperative patients include an increase in metabolic rate of up to 400%, hypoxemia, wound dehiscence, dental damage, and disruption of delicate surgical repairs. The etiology of spontaneous post-anesthetic tremor is most commonly attributed to normal thermoregulatory shivering in response to intraoperative hypothermia. However, the mechanism of this tremor remains unknown, hampering prevention and treatment. The present study was designed to determine whether mechanisms other than thermoregulation contribute to the tremor. The electromyograms (EMGs) of eight muscles were observed in nine women during recovery from isoflurane anesthesia. Signals from each muscle were compared to those of pathologic clonus induced by plantar flexion in unanesthetized patients with spinal cord transections and to those of cold-induced shivering in normal, unanesthetized subjects. Two distinct EMG patterns were identified: 1) regular, bursting signals of 5–7 11z similar to those produced by pathologic clonus in patients with spinal cord transertions; and 2) tonic, irregular signals of 5–15 Hz which had poorly defined hursts that did not demonstrate the synchronous 4–8-cycle/min waxing and waning pattern typical of normal shivering. EMG activity occurred most often at expired isoflurane concentrations of 0.1–0.19%, and was not related to rectal temperature. During the later part of recovery when isoflurane concentrations were ±0.1%, hypothermic patients frequently demonstrated no clinical or EMG evidence of muscular activity. These data suggest that during recovery from general anesthesia: 1) the normal thermoregulatory responses to cold are inhibited, and 2) generalized tremor is a combination of spontaneous clonus and a tonic activity that differs from normal shivering. Neither observation is consistent with the conventional theory that post-anesthetic tremor represents a classical thermoregulatory response.