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 (μg/24 h) with increasing age (P < 0.025 to < 0.001), and the opposite was seen when values were expressed as μg/g creatinine (P < 0.05 to < 0.025). However, when values were expressed as μg/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 μg/m2/24 h; b) the excretion of these compounds increased at a faster rate approximately after 130.0—135.0 cm 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.