Adrenaline, noradrenaline, lactate and glucose levels in the blood, together with the heart rate and oxygen intake were examined in eight boys (12.8 +/- 0.8 years) and seven adults (27.8 +/- 2.9 years) during a graduated treadmill exercise. At rest, noradrenaline is higher in the adults, while adrenaline, lactate, glucose, heart rate and relative oxygen intake show no differences between the groups. At the same exercise levels, adrenaline (+ 90-180%), noradrenaline (+ 28-77%) and the heart rate are higher in boys, corresponding to a higher relative exercise load, and glucose, lactate and the relative oxygen intake show no differences. During maximum treadmill exercise adrenaline, glucose and the relative oxygen intake show no differences between the groups, whereas the noradrenaline (-30%) and lactate levels (-25%) are lower and the heart rate is 4% higher in the boys. There is an identical increase in lactate and catecholamine levels (r = 0.92 and 0.87) with a lower correlation with high intensity "anaerobic" physical exercise, which shows no age dependent difference until about the age of 40. The maximum catecholamine (adrenaline and noradrenaline) and lactate concentration is 25% lower in the boys; an indication of reduced maximum sympathetic activity and reduced maximum anaerobic capacity is seen. The constant relationship between adrenaline and noradrenaline during physical work (r=0.90 and 0.85), also with a lower correlation at high intensive physical exercise, changes from 1 : 3.5 (boys) to 1 : 5.5 (adults) (p less than 0.001), based on higher noradrenaline levels in the adults at rest, at submaximum exercise levels and during maximum ergometric work; these changes are seen to be the cause of an age dependent negative chronotropic effect.