Sustained exercise to fatigue elicits no major differences either in plasma amino acid levels or in brain 5-hydroxytryptamine (5-HT) metabolism between sedentary and endurance-trained animals. Furthermore, 11 weeks of endurance training did not influence the maximal activity of the enzyme monoamine oxidase in the brain areas which were studied. In both sedentary and endurance-trained rats, sustained running to fatigue caused an increase in the plasma concentration ratio of free tryptophan/other large neutral amino acids and an increase in the concentration of tryptophan in the six brain areas that were studied. The increase was similar in the different regions of the brain and averaged 36%. Exercise caused an increase in the levels of 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) in the brain stem (14 and 44% respectively) and hypothalamus (16 and 17% respectively) and an increase in the level of 5-HIAA in the hippocampus (21%) and striatum (28%). Exercise also caused an increase in the level of dopamine in the brain stem (56%) and hypothalamus (46%) and of nor adrenaline in the striatum (59%). Since the levels of 5-HT and dopamine were both increased in the brain stem and hypothalamus, it is possible that these changes may play important roles in the central effects of exercise, including both physical and mental fatigue and effects on mood.