The effects of carbohydrate (CHO) restriction on the hypoglycemic phase of the glucose tolerance test were studied in ten normal subjects. The mean nadir plasma glucose was 64 ± 4 mg/dl (x ± SEM) for the control test, and 48 ± 4 mg/dl (P < 0.01) after 3 days of an isocaloric low CHO diet. Following the low CHO diet, six of ten subjects had a nadir plasma glucose less than 50 mg/dl, and five of these six had mild symptoms of hypoglycemia compared to no biochemical or symptomatic hypoglycemia during the control test. Hormone secretory patterns under the two experimental conditions were measured. CHO restriction produced a significant decrease in early insulin release followed by excessive insulin relative to the control test at 3-4 h of the test. Glucose ingestion produced a depression of plasma glucagon from fasting levels during the control test, which was impaired following CHO restriction. Plasma growth hormone and cortisol responses were not different under the two experimental conditions. These studies demonstrate that CHO restriction followed by concentrated CHO ingestion produces hypoglycemia in normals. They emphasize the need to consider dietary history in evaluation of hypoglycemia. CHO restriction may provide a useful model for further study of the mechanisms of hypoglycemia.