A method for measuring the transit time of a meal, containing sausages, mashed potato, baked beans, and a pineapple custard dessert, through the gastrointestinal tract was evaluated in 14 healthy volunteers. Gastric emptying was determined by incorporating a radioactive marker in the meal and counting over the surface of the stomach using a crystal scintillation detector. Small intestinal transit time was determined by measuring breath hydrogen excretion and by estimating the radioactivity over the cecum. Finally, whole gut transit time was measured by incorporating radiopaque plastic markers or carmine red in the meal and estimating the appearance of these markers in the stool. Our results showed that measurements of small intestinal transit time were reproducible and in the majority of subjects the increase in hydrogen excretion occurred at the same time as the increase in radioactive counts over the surface of the cecum. The passage of the first marker in the stool coincided with the appearance of carmine red. There were no significant correlations between small intestinal transit time and whole gut transit time or the half time for gastric emptying. Incorporation of 10, 25, and 40 g lactulose into our standard meal in place of sucrose increased the rate of transit through the small intestine but did not significantly alter the rate of gastric emptying or the whole gut transit time. Total stool weight for 48 hr after ingestion of the meal was inversely related to whole gut transit time but not to small intestinal transit time suggesting that the tendency to develop diarrhea in response to a meal containing unabsorbable carbohydrate depends more on the lack of colonic accommodation than on the rate of small intestinal transit. Finally, there was no significant correlation between the measurements of small intestinal transit time after a drink of lactulose and the transit time of a meal in the same subjects.