Circulating levels of immunoreactive insulin were measured in healthy subjects and newly diagnosed diabetic patients after the injection of regular and NPH (isophane) insulin. The peak concentration of serum insulin was 26.2 ± 2.9 μU/ml 60 min following the administration of 10 U sc regular insulin. After injection of NPH insulin (10 to 50 U sc), peak serum insulin levels occurred between 2 and 9 hr and the level was significantly correlated with the injected dose. There was marked variation in the time of occurrence of the peak insulin level and its absolute concentration in individual subjects. The serum insulin patterns observed in this study contrast with the previously reported endogenous insulin patterns measured in healthy subjects during a 24 hr period (1), and indicate one reason for the difficulty in maintaining normal blood sugar concentrations in insulin-deficient diabetic patients.