Several aspects on the effect of nutrients on small intestinal motility are not completely understood. We have analyzed changes of motor activity of the canine small bowel following intragastric administration of casein and soy protein. Studies were performed by intraluminal manometry with external transducers and multiple infused catheters with closely spaced registrating sites. Casein was followed by a statistically significant decrease of amplitude and frequency of small intestinal contractions, compared to soy protein. Pretreatment with naloxone suppressed the inhibitory effect of casein, suggesting that stimulation of opioid receptors by beta casomorphins, a product of digestion of casein, might be involved in the motility changes observed.