This study was undertaken to determine whether fat leaves the stomach within or bound to the surface of particles of solid food. We studied gastric emptying of fat in 6 human subjects and in 6 dogs with Roux-en-Y duodenojejunostomies so that chyme leaving the stomach could be collected free of bile and pancreatic enzymes. Humans were studied with a duodenal multilumen tube so that phase-specific, nonabsorbed markers in the meal could be tracked with corresponding, phase-specific markers perfused into the duodenum. In this way, we observed the separate time-courses of gastric emptying of the aqueous and solid phases, as well as the extracellular fat (ECF) and intracellular fat (ICF) phases of the meal. In the dogs, all chyme leaving the stomach was collected from a Roux-en-Y fistula and was analyzed directly for aqueous, solid, ECF, and ICF markers in the meal. In both the humans and the dogs, the aqueous phase emptied promptly, whereas the solid, ECF, and ICF phases emptied together, in parallel, after an initial lag. In humans, 22% of the ECF versus 51% of the ICF (p less than 0.02) emptied on or in the solid food particles. In dogs, 81% of the ECF emptied as an oil, 13% emptied on the solid particles, and only 6% emptied as a stable, aqueous emulsion. Sixty-six percent of the ICF emptied in the solid food particles, 20% as a stable, aqueous emulsion, and 14% as an oil. We conclude that most of the ICF empties within the solid food phase, whereas most of the ECF empties as an oil phase.