In 6 type 1 diabetic patients treated with a basal-prandial injection regimen with a pen, plasma concentrations of free insulin and glucose were measured over a period of 24 h. We obtained an insulin peak 1 h after injection of regular insulin and lower levels at night. Between peaks, basal levels of free insulin were not reached. Glucose concentrations fluctuated only slightly. In 10 healthy volunteers, the absorption rate of insulin was studied after subcutaneous injection of 125I-Actrapid using a conventional technique and with a pen. During an 8-h period residual radioactivity and plasma insulin and glucose were measured. No significant differences were found, except for the insulin concentration 60 min after injection (higher after pen injection, P less than 0.05). The variability of the parameters was greater following pen injection (P less than 0.02). A basal-prandial injection regimen can thus produce stable glucose levels but only with considerable hyperinsulinaemia. Injection with an insulin pen causes a modest increase in absorption rate and plasma concentrations of insulin.