Simultaneous recordings of the effect of cold exposure on pulsatile cerebral impedance (PCI), heart rate (HR) and available oxygen (aO2) in the rat confirmed the conclusion, arrived at by recording (PCI + HR and aO2 separately, that the brain participates in thermoregulatory heat production. For statistical analysis of changes in cerebral metabolic rate (CMR) from experiments in which either only PCI + HR, or aO2 were recorded, changes in PCI and HR, i.e. in cerebral flood flow (CBF), and in a aO2 were combined into two indices (Index CMR/A and Index CMR/B) to indicate changes in CMR. Both indices demonstrated highly significant increases in CMR in response to cold in the rat and the young rabbit. In the rat, the magnitude of the response depended on stimulus intensity and on the distance from the surface of the brain, being greater near the surface than at greater depths. Circumstantial evidence suggests that the main source of thermoregulatory heat production in the brain has to be sought in the neuroglia.