The daily urinary excretion of dopamine (DA), noradrenaline (NA) and adrenaline (A) was investigated in 43 clinically healthy males and 37 females, aged 13 days to 37.5 years. There was a progressive increase in the daily output of each catecholamine (\g=m\g/24 h) with increasing age (P < 0.025 to <0.001), and the opposite was seen when values were expressed as \g=m\g/g creatinine (P < 0.05 to <0.025). However, when values were expressed as \g=m\g/m2/24 h no significant differences were disclosed among the different age groups, except for a transient rise in DA excretion in the group 1.0-4.9 years. A significant linear correlation was observed between chronological age and urinary DA (r = 0.814, P < 0.05), NA (r = 0.837, P < 0.05) and A (r = 0.839, P < 0.05), and also between body weight and each one of the catecholamines (DA: r = 0.731, P <0.05; NA: r = 0.839, P < 0.05; A: r = 0.720, P < 0.05). However, a quadratic correlation existed between height and DA (r = 0.759, P< 0.05), NA (r = 0.853, P< 0.05) and A (r = 0.814, P < 0.05). Each one of these models could explain up to 73% of the observed variability. It is suggested that: a) the most convenient form to express the daily urinary excretion of catecholamines is as \g=m\g/m2/24 h; b) the excretion of these compounds increased at a faster rate approximately after 130.0\p=m-\135.0cm of height; c) this height-related greater increase in the output of catecholamines occurs simultaneously with previously reported increases in haemoglobin and haematocrit and significant changes in plasma concentrations of thyroid hormones; d) although the mechanisms involved in these changes remain to be clarified, it is interesting to note that they occur at a time when oxygen requirements are greater consecutive to the rapid increase of lean body tissues.Quantitation of the daily urinary excretion of cate¬ cholamines and their metabolites is of great value in the diagnosis and management of neuroblastoma, ganglioneuroma and phaeochromocytoma.Nevertheless, data on the urinary excretion of dopamine (DA), noradrenaline (NA) and adre¬ naline (A) in healthy children is scanty and incom¬ plete (Kärki 1957;Cession-Fossion et al. 1964;Voorhess 1967;Comoy & Bohuon 1970;Borrell et al. 1974;De Schaepdryver et al. 1978). Further¬ more, the normal data available are difficult to compare to each other, not only because of the different methodology used, but also because of the lack of uniformity in expressing the results. The urinary excretion of catecholamines (as ng) has been referred to as an absolute value in 24 h, or per m2 of body surface, per kg body weight or per g of urinary creatinine. Each investigator has claimed reasons in favour of each form of express¬ ing the results. However, it is clear that: 1) chrono¬ logical age and body weight have a moderate importance (at best) as indicators of biological age,