Erythromycin (EM), discovered in 1953, was the first clinically useful macrolide antibiotic, widely used against Gram-positive bacteria, Gram-negative cocci, and mycoplasmas. Recently, EM was found to stimulate gastrointestinal tract activity resembling normal interdigestive motor activity. We report that some EM derivatives obtained in our laboratories have in vivo gastrointestinal motor stimulating (GMS) activity 60-2890 times stronger than that of erythromycin A (EM-A) while the antibacterial activities are lost. Based on findings that EM's GMS activity is specific to 14-membered macrolides and mimics motilin at low doses, we conducted chemical modification of EM to obtain derivatives with stronger GMS activity and no antibacterial activity for modulating gastrointestinal disorders. In vivo GMS activity was measured via chronically implanted force transducers on the gastrointestinal serosa of fasted conscious dogs, and in vitro studies used rabbit duodenum muscle strips. Among EM-A derivatives (e.g., acyl, sulfonyl, hemiketal, quaternary ammonium derivatives), compounds like 8,9-anhydroerythromycin A 6,9 hemiketal (1, 10-fold activity vs EM-A), its N-ethyl analogue (2, 18-fold), methyl quaternary ammonium derivative (3, 20-fold, no抗菌 activity), and propargyl bromide derivative (6, 2890-fold, no抗菌 activity) showed enhanced GMS activity. Compound 6 induced strong, caudally migrating gastrointestinal contractions in dogs similar to natural interdigestive activity. In vitro, its contractile pattern on rabbit duodenum differed from acetylcholine, and was not inhibited by tetrodotoxin or atropine. These derivatives may be useful for modulating gastrointestinal contractile activity and studying motility physiology.