8 subjects were exposed to the Stroop mental performance test in a design with alternating hourly periods of rest and stress. During each period one urine sample and several venous plasma samples were obtained. Heart rate responded rapidly to initiation and termination of the stress exposure with increases and decreases respectively. Both urinary and plasma adrenaline increased significantly during stress. The plasma response was immediate and sustained. Neither urinary, nor plasma noradrenaline were significantly increased by the test. Plasma noradrenaline, however, increased significantly on termination of the exposure to stress. It was suggested that the latter effect may be due to muscle sympathetic nerve activity decreasing during stress and increasing following stress. The sample-to-sample variation was more than 20% of the mean for both catecholamines, indicating the need for frequent sampling to reliably reflect plasma levels. The mean intraindividual plasma/urine correlation was r = 0.70 (p less than 0.001) for adrenaline and r = 0.40 (p less than 0.05) for noradrenaline. When only resting periods were considered, no significant correlations remained, apparently due to a reduced range of variation and accompanying reduced signal-to-noise ratio. It is concluded that both urinary and plasma adrenaline may be useful in the evaluation of changes in sympatho-adrenal activity during stress.