Peripheral blood flow was measured during periods of good and poor metabolic control in juvenile diabetics with less than five years duration of the disease. Previously insulin treated patients, in whom the insulin was withdrawn for a few days, showed elevated blood flow in muscular, adipose and cutaneous tissue, whereas a group of newly diagnosed diabetics before treatment showed elevated blood flow in adipose and cutaneous tissue only. In good metabolic control the diabetics had normal values of both muscular, adipose and cutaneous tissue blood flow. In poorly controlled diabetics blood pressure, pulse rate and body temperature were elevated. These parameters are also normalized when the metabolism is brought under control. It is suggested, that the increased peripheral blood flow seen in diabetics in poor control is the effect of a generally increased turnover of energy in combination with local factors, different from tissue to tissue.