Metabolic adjustment was studied in male Wistar rats weighing 180-200 g with bilateral electrolytic lesions in the preoptic area which were acclimatized to a hot environment (34 degrees C, 30 rats) and to normal temperature (25 degrees C, 20 rats) for 60 days. Oxygen consumption at 34 degrees C and body weight were measured periodically. Preoptic-lesioned rats (PO rats) showed a reduction in metabolic rate (from 46.2 +/- 2 to 33 +/- 2 Cal/m2 per h, P less than 0.05) regardless of the temperatures to which they were adapted. Although the reduced metabolic rate of PO rats living at 25 degrees C or 34 degrees C could be an indicator of adaptation to heat, only the PO rats living at 25 degrees C showed a weight gain (101 +/- 13 g) similar to that observed for the sham-operated control (75 +/- 11 g). In fact, the PO rats living at 34 degrees C failed to increase their body weight during 60 days of exposure to heat. These data indicate that the preoptic area is essential to integrate the thermal input with the energy-linked metabolic process involved in adaptation to hot environments.