The cold-adaptation process was examined in preoptic-lesioned (PO, 18 rats) and adrenaldemedullated-preoptic-lesioned rats (ADPO, 16 rats). PO rats and ADPO female Wistar rats were cold acclimatized to 5 degrees C for 2 operated and then treated exactly like the lesioned rats. Lesions in the PO region caused a decrease in the ability to maintain normal body temperature in a cold environment (5 degrees C for 2 h). Their colonic temperature fell even by the end of the 2nd h of cold exposure (-1.1 degrees C for PO rats and -1.25 degrees C for ADPO rats, P less than 0.01). However, after 2 weeks of cold acclimation the PO rats, but not the ADPO rats were able to thermoregulate in the cold and showed a normal increase in interscapular brown adipose tissue weight (79%, P less than 0.01). These data suggest that the PO is not an essential link for cold adaptation when a mild cold-adaptation method is used. Furthermore, the data show that increased adrenal medullary activity is required for the cold adaptation process in PO rats.