The effect of aqueous extracts of Stevia rebaudiana leaves on a glucose tolerance test was investigated in 16 normal volunteers. Aqueous extracts of 5 grams of leaves were administered to volunteers at regular 6-h intervals for 3 days. Glucose tolerance tests were performed before and after extract administration. A second group of 6 normal volunteers who ingested an aqueous arabinose solution was also studied to eliminate possible stress effects. The extract of Stevia rebaudiana increased glucose tolerance. The extract significantly decreased plasma glucose levels during the test and after overnight fasting in all volunteers.