ABSTRACTSIn this study, we investigated whether capillary plasma catecholamines can be used as a suitable substitute for arterial catecholamines. Analysis was done radioenzymatically. Catecholamine concentrations were not different in arterial and simultaneously collected “arterialized” (warmed foot) capillary plasma obtained by heel-prick from 18 neonatal intensive care patients as assessed by linear regression analysis (correlation coefficient: 0.966 for noradrenaline; 0.894 for adrenaline; p < 0.05) and by a Wilcoxon test [noradrenaline: 2.13 (0.61–10.47)versus2.41 (1.05–10.23); adrenaline: 0.75 (0.16–1.70)versus0.71 (0.10–1.37) nmol/L, median (range)]. However, “arterialization” of capillary blood is important; when blood was obtained in nine neonates without warning their feet, capillary concentrations of noradrenaline were higher than arterial values (p < 0.03) and those of adrenaline were not different from arterial values. Catecholamne concentrations in arterialized capillary plasma collected in healthy full-term infants at 1 h [n = 9; noradrenaline: 6.85 (3.49–8.88) nmol/L; adrenaline: 1.34 (0.86–2.85) nmol/L and 5 d after birth [n = 27; noradrenaline: 1.58 (0.89–3.16) nmol/L; adrenaline: 0.59 (0.25–1.64) nmol/L] reflect the well-known fall (p < 0.01) in catecholamine levels after delivery. With a highly sensitive analytical technique, catecholamine concentrations can reliably be assessed in minute samples (100–200 μL) of arterialized capillary blood, even when concentrations have dropped to low “resting” basal levels. Moreover, the capillary sampling procedure is simple and safe, can easily be applied to healthy infants, and does not have the practical and ethical limitations of arterial blood sampling. (Pediatr Res31: 579–582, 1992)