Glucose tolerance and intensive islet stimulation tests were done on eighty-seven subjects who covered a broad range of age and glucose tolerance. The data suggest decreasing glucose tolerance with increasing age compatible with the observations of others. Over a broad range of glucose tolerance there was seen such a broad spread of serum immunoreactive insulin levels (IRI) that it was not possible to relate them directly to the degree of glucose tolerance seen. This suggested that multiple factors, including variations in peripheral resistance to insulin not attributable to known factors, are operative in determining glucose tolerance. Only when glucose tolerance was very abnormal or fasting blood sugar was elevated was there apparent decrease in serum IRI response to the stimuli used, making it difficult to attribute mild-to-moderate degrees of glucose intolerance to lack of capacity for IRI release. A close correlation between the two-hour IRI in a glucose tolerance test and the levels obtained after the stimulus of glucagon and tolbutamide suggested that the former may be an adequate reflection of islet secretory capacity.