The nature of muscle efferent fibre activation during whole body cooling was investigated in urethane-anaesthetized rats. Multiunit efferent activity to the gastrocnemius muscle was detected when the trunk skin was cooled by a water-perfused jacket to below 36.0 ± 0.7°C. That efferent activity was not blocked by hexamethonium (50 mg kg−1, I.V.) and was not associated with movement or electromyographic activity. Cold-induced efferent activity enhanced the discharge of afferent filaments from the isotonically stretched gastrocnemius muscle, demonstrating that it was fusimotor. Fusimotor neurons were activated by falls in trunk skin temperature, but that activity ceased when the skin was rewarmed, regardless of how low core temperature had fallen. While low core temperature alone was ineffective, a high core temperature could inhibit the fusimotor response to skin cooling. Fusimotor activation by skin cooling was often accompanied by desynchronization of the frontal electroencephalogram (EEG), but was not a simple consequence of cortical arousal, in that warming the scrotum desynchronized the EEG without activating fusimotor fibres. Inhibition of neurons in the rostral medullary raphé by microinjections of glycine (0.5 M, 120–180 nl) reduced the fusimotor response to skin cooling by 95 ± 3%, but did not prevent the EEG response. These results are interpreted as showing a novel thermoregulatory reflex that is triggered by cold exposure. It may underlie the increased muscle tone that precedes overt shivering, and could also serve to amplify shivering. Like several other cold-defence responses, this reflex depends upon neurons in the rostral medullary raphé.