Adrenaline and noradrenaline concentrations in arterial and simultaneously collected "arterialised" capillary plasma (vasodilated ear-lobe) obtained from unmedicated resting subjects (n = 9) were measured radio-enzymatically and found to be indistinguishable (adrenaline: 112 +/- 42 ng/l versus 109 +/- 52 ng/l; noradrenaline: 378 +/- 174 ng/l versus 410 +/- 219 ng/l; mean +/- SD; paired t-test: P greater than 0.10). The fitted regression lines did not differ significantly from the line of identity (slope = 1). The correlation coefficient was 0.969 for adrenaline and 0.945 for noradrenaline. Adrenaline and noradrenaline concentrations in plasma from freely flowing capillary blood (non-hyperaemic fingertip) were very similar to those in arterial plasma (adrenaline: 101 +/- 89 ng/l versus 90 +/- 72 ng/l;noradrenaline: 399 +/- 240 ng/l versus 395 +/- 240 ng/l; n = 13 each; paired t-test: P greater than 0.10). The correlation coefficient was 0.981 and 0.917 for adrenaline and noradrenaline, respectively. Adrenaline concentrations in capillary plasma (vasodilated earlobe), however, were significantly higher than those in venous (forearm) plasma (100 +/- 51 ng/l versus 61 +/- 23 ng/l; n = 15; P less than 0.01; capillary-venous difference = 35 +/- 18%) while noradrenaline concentrations did not differ significantly. We conclude that capillary plasma resembles arterial plasma in its catecholamine content and we show that arterial catecholamine levels may be determined in capillary, especially "arterialised" specimens, eliminating the need for arterial blood sampling.