Norepinephrine (NE) and serotonin (5-HT) concentrations were determined in three areas of the rat brain following 8 wk exposure to differing physical activity and dietary regimens. Animals were divided into sedentary and exercise groups, and one-half of each received either a normal or atherogenic diet. The exercise consisted of 30 min of treadmill running, 5 days/wk, for 8 wk. Brains were weighed and sectioned into cerebral cortex, cerebellum, and remainder (midbrain), and fluorometrically analyzed for NE and 5-HT content. In most brain areas, NE and 5-HT levels were significantly greater among exercise-normal diet and exercise-fat diet compared to both sedentary groups. In the cerebral cortex, exercise-normal diet rats possessed higher levels of NE and lower concentrations of 5-HT than sedentary animals. The atherogenic diet did not affect amine levels other than 5-HT in the cortex among exercise rats. The NE results are consistent with prior research indicating its control of sympathetic function. Increased levels of 5-HT in the midbrain may be the neurotransmittal adaptation responsible for decreased appetite and enhanced weight loss following chronic endurance exercise.