To examine the role of monoamines and amino acids in thermoregulation, we measured their concentrations in the preoptic area and anterior hypothalamus (PO/AH) in exercising rats, using an in vivo microdialysis technique. Body temperature (Tb) was monitored using a telemetry system. Tb increase by about 1.0 degrees C in the first 15 min of treadmill exercise (10 m/min; for 60 min), and was maintained thereafter at a steady high level possibly due to activation of the heat loss system. The levels of dopamine metabolites (3,4-dihydroxyphenylacetic acid and homovanillic acid) in the PO/AH significantly increased during exercise. However, exercise did not induce an increase in the level of either serotonergic substances (5-hydroxytryptamine and 5-hydroxyindoleacetic acid) or amino acids (aspartate and glutamate). Our data indicate that dopamine breakdown processes in the PO/AH are activated during exercise. Dopamine in the PO/AH may be involved in the heat loss mechanisms for thermoregulation when Tb rises during exercise.