Summary Blood glucose, serum free fatty acids,-insulin, -glucagon and -growth hormone have been measured half-hourly in five newly diagnosed, untreated, male patients with classic juvenile diabetes and in five healthy male subjects during a 24-h period of daily life. —Blood glucose, serum insulin and -free fatty acids followed, on the whole, the expected pattern. Serum gluoagon showed a fairly constant level during day and night in both groups. — In the non-diabetic subjects, serum growth hormone was low during most of the day. Only two peaks were observed before 10 p.m. Four of the subjects showed peaks at precisely the same time, namely at 10.30 p.m. and 1.30 a.m. Two showed peaks at 5.00–5.30 p.m. The mean serum growth hormone concentration during the 24-h period was 1.98 ng/ml. — In the juvenile diabetics, the growth hormone was higher and the level fluctuated much more, showing more frequent and higher peaks than in the non-diabetics. The mean serum growth hormone concentration during the 24-h period was 7.26 ng/ml, i.e. three to four times higher than in the non-diabetics.