It is generally accepted that the intestinal phase of pancreatic secretion is initiated by the stimulation of chemoreceptors sensitive to fat and protein degradation products and hydrogen ions. The effect of the volume and osmolality of food emptied by the stomach into the duodenum has received less attention. We investigated the effects of these factors on the stimulation of pancreatic secretion by studying 8 healthy male volunteers (ages 23-69 yr), in random order on 3 separate days. On day 1, an amino acid mixture (L-phenylalanine, L-tryptophan) was infused intraduodenally at increasing rates, 0.2, 0.8, and 3.2 ml X min-1. On day 2, normal saline was infused into the intestine at the same increasing rates. On day 3, mannitol solutions of increasing osmolality 370, 520, and 700 mosmol X kg-1 were infused into the duodenum at 0.2 ml X min-1. Duodenal contents were continuously aspirated via a double-lumen tube and PEG 4000 was used as a recovery marker. All studies were repeated 90 min later during i.v. infusion of atropine (20 micrograms X kg-1 X h-1). Increasing volumes of amino acids significantly increase amylase and bicarbonate output (p less than 0.05) in a stepwise fashion. Increasing volumes of saline also caused a similar stepwise increase in amylase and bicarbonate output. Furthermore, increasing osmolality caused an increase in enzyme output up to 520 mosmol X kg-1 and no increase was seen thereafter. The responses seen with volume and osmolality were approximately 40% of that obtained with the amino acids. All responses were significantly reduced (p less than 0.05) during atropine infusion. We conclude that the human duodenum contains receptors for volume and osmolality that stimulate both pancreatic enzyme and bicarbonate secretion. Both mechanisms are atropine sensitive, suggesting they are mainly neurally mediated.