To investigate the effect of site and timing of insulin injection on post-brekfast plasma glucoseconcentration, 23 children with type 1 diabetes were given their usual mixture of short- and intermediate-acting insulin five minutes prior to breakfast on one day and between 15 and 60 minutes before breakfast on the alternate day, depending on their 7:00 am fasting blood glucose concentration. Thirteen children received insulin in an extremity, and 10 in the abdominal wall. Plasma insulin and glucose patterns were similar for the two injection sites. The mean post-breakfast peak glucose increment was significantly lower and the mean increment in free insulin values during the first hour after breakfast was higher on the day when insulin preceded breakfast by 30 to 60 minutes. Thus, significant reductions in post-breakfast hyperglycemia can be achieved by increasing the interval between the injection and breakfast in proportion to the fasting capillary glucose concentration. The timing of the morning insulin injection may be more important than the injection site.