Brain serotonin has long been implicated in the regulation of body temperature, although its precise role is not completely understood. The present study examined the effects of environmental cooling (4-8 degrees C for 2 or 4h) on the single-unit activity of serotonergic neurons recorded in the medullary raphe nuclei obscurus and pallidus and in the pontine dorsal raphe nucleus of freely moving cats. These neuronal groups have primarily descending projections to the spinal cord and ascending projections to the forebrain, respectively. Cold exposure induced shivering and piloerection, but no appreciable changes in core temperature. Of the medullary serotonergic cells studied (n=14), seven were activated and seven were unresponsive to cold exposure. For the responsive cells, the mean increase and peak effect in unit activity relative to baseline were 31% and 46%, respectively. Of the seven cold-responsive cells, the activity of four was monitored when the animals were transferred back to room temperature (23 degrees C). Within 15-30 min, the activity of these cells returned to baseline. In contrast, none of the dorsal raphe nucleus cells studied (n=14) displayed a significant change in neuronal activity during cold exposure, suggesting that these neurons do not receive afferent input from cold-sensitive cutaneous receptors or participate in thermoregulatory responses evoked by low ambient temperatures.Overall, these results suggest that a subset of medullary serotonergic neurons play a role in physiological mechanisms underlying cold defense (e.g. increases in motor output and/or autonomic outflow). On the other hand, the lack of responsiveness of serotonergic dorsal raphe nucleus neurons to cold exposure does not support a specific role for these cells in thermoregulation.