PURPOSE: We studied smooth muscle strips from rabbit gastric antrum and low esophageal sphincter (LES) to explore the difference between newborn and adult rabbit on erythromycin (EM)- induced contractions. Another aim of this study was to determine the mechanism of LES contractile activities induced by erythromycin (EM). METHODS: Muscle strips prepared from newborn and adult rabbits were stimulated with agonists such as KCl, motilin and EM, and the isometric tensions were measured. To study the underlying mechanism of EM-stimulated contraction of LES, the receptor antagonsts, including tetrodotoxin, hexamethonium, atropine, propranolol and phentholamine, were used. A high concentration of motilin (1 micrometer) was employed to provoke tachyphylaxis. RESULTS: Antral smooth muscle and LES from newborn rabbits developed less active force than those from adult rabbits when stimulated with KCl, motilin and erythromycin, however, the effective concentrations (EC50s) were similar in both age groups. Antral smooth muscle strips showed both tonic and phasic contractions but LES muscle strips showed only tonic contractions. These findings were observed in both newborn and adult rabbits. The contraction force of antral smooth muscle strips in response to agonists was greater than that of LES. The contractile response of LES to repeated motilin or EM administration was markedly decreased. EM- induced contractions of LES were markedly decreased by motilin tachyphylaxis but were unaffected by tetrodotoxin, hexamethonium, atropine or propranolol plus phentolamine. CONCLUSOIN: The data suggest that the contractilities of antrum and LES smooth muscle from newborn rabbit are less than those from adult ones, however, the effective concentratons of EM (EC50) are not different between the two age groups. The results also suggest that erythromycin induces the contraction of rabbit LES via motilin receptor in vitro.