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Responses of normothermic and hibernating marmots to manipulations of the preoptic-hypothalamic temperature (TPO) were studied. Independent variables included alteration of TPO and, during normothermia, room temperature. Hibernation occurred at an ambient of 6 degrees C. Dependent variables include brain, subdermal, and surface temperatures, heart rate, and behavioral, electromyographic cortical, and hippocampal responses. Although normothermic autumn marmots displayed most of the usual mammalian responses to alterations of the TPO, evidence of effective dermal vasomotion was not obtained. Single episodes of water drinking accompanied prolonged raising of the TPO; sleep was not elicited. During hibernation, effective central thermoregulation was not apparent until 3 or 4 days had elapsed. After this, thermoregulation was readily demonstrable in response to both raising and lowering the TPO. The apparent open-loop gains (OLG) for rise in body temperature after lowering of the TPO showed an exponential increase in value at lower prestimulus body temperatures. It was postulated that this could be explained on the basis of the recruitment of cold-sensitive neurones, which in turn would provide an explanation for the hypothesized "alarm temperature."