A test meal containing emulsified coconut oil mixed with water soluble polyethyleneglycol (PEG) of high mol. wt. was given to rats by gastric intubation. At intervals up to 5 hr. after the meal, rats were killed and the amounts of PEG and fat recoverable from the stomach, small intestine and coecum were measured. PEG was not absorbed from, nor broken down in, the intestine and left the stomach at the same rate as the emulsified fat. It was, therefore, a suitable reference substance. With a test meal of constant volume, 4 ml., increasing doses of fat, 0-25, 0.5 and 140 ml., decreased the rate of stomach emptying and of propulsion of PEG through the small intestine. The rate of fat absorption was limited by the decreasing rate of stomach emptying but there was no indication that the absorptive capacity of the small intestine had been saturated. The ratio of recovery of fat: PEG in successive segments of the small intestine indicated that fat was absorbed chiefly in the distal half. This was probably due to rapid propulsion of material through the proximal half rather than to any lack in capacity for fat absorption proximally.WHEN a suitable reference substance has been added to a test meal, it is possible to follow the progress of the meal through the intestine and to assess the extent to which foodstuffs are absorbed from a given segment. The reference substance must be biologically inert and unabsorbable. It also must pass along the intestine at the same rate as the material whose absorption is being investigated.Reference substances have been used mostly in experiments on watersoluble foodstuffs [Reynell and Spray, 1956 a]. Their use with fat presents difficulties, because the changes which fat undergoes during digestion and absorption are not easily reproduced with an inert, unabsorbable substance. Borgstrom, Dahlquist, Lundh and Sjovall [1957] have used, in human subjects, polyethyleneglycol (PEG) of mol. wt. 3000-3700. They considered that this was a reliable reference substance, despite its high solubility in water, when the fat in the test meal was finely emulsified. PEG therefore seemed worthy of trial in experiments on fat absorption in rats. The objects of these experiments were: (i) to see how the rate of passage of a gastric test meal or its residues through the small intestine was affected by increasing the dose of fat; (ii) to assess the relative importance of proximal and distal segments in fat absorption, and (iii) to see whether the rate of fat absorption was limited by the absorptive capacity of the small intestine or by the rate at which fat was admitted from the stomach.

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