The gastric emptying characteristics of physiological saline (0.9% NaCl) and glucose solutions of three different concentrations (0.05, 0.125, 0.25 g/ml) were examined in order to identify distinctions in the control of the stomach's activity. Saline emptied rapidly and exponentially. Glucose assumed, soon after filling the stomach, a slow and calorie-constant emptying pattern such that 2.13 kcal of glucose were delivered per minute to the duodenum for all three concentrations of glucose. When, by means of a catheter passed beyond the pylorus, glucose was infused into the duodenum in amounts varying from 26.5 to 120 kcal, an inhibition on the gastric emptying of physiological saline of 0.46 min/kcal of intraduodenal glucose was demonstrated. Since 2.13 kcal/min and 0.46 min/kcal are reciprocals, it appeared that in emptying saline, the gastroduodenal system acts as an "open-loop" system passing liquids from the stomach at a rate primarily determined by the volume of gastric contents. With glucose, however, a "closed-loop" system is established that assumes a steady-state balance between the delivery of glucose to the duodenum and the inhibition of this delivery evoked from the duodenum by the glucose that enters it.