Motility and exocrine and endocrine secretions of the gastrointestinal tract are modified during postprandial periods. Experimental evidence suggests that nutrients stimulate small-intestine absorption, but the mechanisms involved are uncertain. Further, the effects of mixing nutrients with different physicochemical compositions on absorption and motility are not completely elucidated. With the use of a canine model, we investigated the effects of a mixture of different lipids and proteins on small-intestine motility and absorption. Two cannulae, one in the proximal duodenum and one in the distal ileum, were implanted in the dogs. Absorption of a continuous duodenal infusion of eight test mixtures consisting of long- and medium-chain fatty-acid triacylglycerols, soy protein and its hydrolysate, casein and its hydrolysate, and variable amounts of glucose, and glucose polymer were analyzed by measuring the volume, concentration, and total amount of triacylglycerols, nitrogen, and glucose recovered from the ileal cannula. Significant differences in absorption were observed across test mixtures. Significantly lower absorption of H(2)O and nutrients was observed with test mixtures prepared with medium-chain fatty-acid triacylglycerols and soy protein compared with test mixtures prepared with corn oil, casein, and hydrolysates of both proteins. We concluded that the physicochemical characteristics of nutrients in elemental diets continuously infused into the duodenum are important for determining small-intestine absorption.