Intraindividual variation in the results of repeated oral glucose tolerance tests in normal subjects is well recognized but incompletely explained. The present studies show that such variation can be produced by ingestion of the glucose solution during different phases of the normal fasting activity cycle of the upper gut. Such variation is not seen when glucose is administered intraduodenally during the same phases of activity. Gastric emptying shows similar variation with the activity cycle; larger volumes of solution were emptied from the stomach during activity than quiescence, thus presenting greater quantities of glucose solution to the small intestine for absorption. Metoclopramide and hyoscine butylbromide, drugs known to influence the rate of gastric emptying, reduced the variation in the glucose tolerance test. The data suggest a possible use of the glucose tolerance test for the assessment of gastric emptying.