Five intensively treated, insulin-dependent diabetics exercised for 45 minutes after fasting while receiving basal insulin injections. Plasma glucose concentrations remained stable during exercise but then declined, resulting in clinical hypoglycemia 1 to 2 hours later. Efficacies of three pre-exercise snacks in preventing the hypoglycemia were compared in a randomized crossover design. Orange juice, whole milk, and skim milk, each containing 13 g of carbohydrate, all prevented postexercise hypoglycemia. However, the more rapidly absorbed snacks, orange juice and skim milk, caused a greater increase in plasma glucose concentrations and the area under the glucose curve during exercise. From the recognized glucose profiles that occur after consumption of different carbohydrates, snacks as well as exercise and insulin can now be programmed for intensively treated, insulin-dependent diabetics. Because plasma glucose levels remain stable during exercise done after fasting and only fall late after exercise, a "lente" carbohydrate snack, such as whole milk, is an appropriate pre-exercise snack.