To investigate changes of free and sulfoconjugated catecholamines in response to alterations in sympatho-adrenal activity, free and conjugated noradrenaline, adrenaline and dopamine were determined radioenzymatically in plasma of 49 subjects. During brief vigorous bicycle exercise (8 min, maximal heart rate: 177 beats/min) mean free noradrenaline and adrenaline values of 2.0 and 0.51 nmol/l at rest, increased to 6.7 and 2 nmol/l (P less than 0.001) respectively, at the maximal workload of 200 watt, whereas conjugated noradrenaline and adrenaline decreased from 3.4 and 0.8 nmol/l to 2.1 and 0.4 nmol/l (P less than 0.001) respectively. In the tenth min of the recovery period basal free and conjugated noradrenaline and adrenaline levels were measured. The moderate stress of a steam bath (20 min, maximal heart rate: 131 beats/min) doubled free noradrenaline and adrenaline levels. However, conjugated noradrenaline and adrenaline concentrations remained unchanged. The increase in free catecholamine values during an exhausting cross-country march over 20 km was associated with an accumulation of sulfated catecholamines. After a rest of 30 min free noradrenaline and adrenaline reached basal values, whereas conjugated noradrenaline and adrenaline remained elevated by 64 and 70% respectively, compared to pre-exercise concentrations. It was concluded that conjugated noradrenaline and adrenaline may be used as pools for free noradrenaline and adrenaline during brief vigorous exercise. In addition, they may also be indicators of chronic activation of the sympatho-adrenal system.