Drugs intended for absorption in the stomach or upper intestinal tract are administered in oral drug delivery systems in conjunction with any of various substances that have been discovered to function as potent agents for inducing the fed mode. By inducing the onset of the fed mode, these agents cause the stomach to prolong its retention of the drug delivery system, which is either large enough to be retained in the stomach during the fed mode or swells or expands to such a size upon ingestion. The fed mode inducing agents include the following compounds and their salts: glycine and glycylglycine, xylitol and related sugar alcohols, sodium and other metal docusates, .beta.-casomorphins, .alpha.-lipoic acid and similarly structured acids, 2,2-diaryl-4-(4'-aryl-4'-hydroxypipendino)butyramides, arginine, Trp-Trp, alkylpyridinium halides, dihydroxybenzoic acids, and potent sweeteners such as aspartame, aspartic acid, acesulfame, and stevioside.