The effect of bovine casein and synthetic beta-casomorphins on the motility of rat gastrointestinal tract was studied by noninvasive techniques using the nonabsorbable marker 141Ce. Casein suspensions (CAS) or whey protein suspensions (WPS) were labeled with 141Ce and fed by gastric tube. Gastric emptying rate (GER) as well as gastrointestinal transit time (GITT) of the tracer were significantly longer with feeding CAS compared to WPS. The differences between the CAS and the WPS groups were partly (GER) or completely (GITT) abolished by pretreating the animals with the specific opiate-receptor antagonist naloxone. It is assumed that opioid peptides released from casein during digestion slowed gastrointestinal motility by direct interaction with gut opiate receptors. To prove whether beta-casomorphins, when given by gastric tube, can affect motility, different synthetic beta-casomorphins in doses between 1 and 10 mg were added to the WPS. The beta-casomorphin-4 (Tyr-Pro-Phe-Pro-NH2) showed no effect on GITT. The D-Ala substituted D-Ala-beta-casomorphin-4 (Tyr-D-Ala-Phe-Pro-NH2) and D-Ala-beta-casomorphin-5 (Tyr-D-Ala-Phe-D-Ala-Tyr-NH2), which are more resistant to proteolytic attack and have higher opioid potency than beta-casomorphin-4, slowed GITT in a dose-dependent manner.