Sufficient gallbladder emptying accelerates early gallstone clearance after extracorporeal shock wave lithotripsy (ESWL). Litholytic therapy with ursodeoxycholic acid (UDC) subsequent to ESWL increases fasting volume (FV) and postprandial residual volume (RV) of the gallbladder. This may lead to retention of cholesterol crystals and small fragments within the gallbladder. In order to find out whether erythromycin, a motilin agonist, improves gallbladder emptying, we tested gallbladder motility after administration of ursodeoxycholic acid with and without oral application of erythromycin. Ten healthy males (age 26-35 years) obtained 10 mg/kg/d of UDCA as a single bedtime dose for three weeks. Prior and after UDCA administration, gallbladder FV was determined sonographically after overnight fasting. After a test meal (490 kcal), gallbladder volume was measured every 5 min until the gallbladder had reached its minimal RV. The next day the same procedure was repeated with 500 mg erythromycin p.o. 45 min prior to test meal application. FV, RV, ejection volume (EV = FV-RV) and ejection fraction (EF = EV/FV x 100) were calculated and differences were compared by the student's t-test. FV (29 ml +/- 8 ml vs. 38 ml +/- 10 ml), RV (12 ml +/- 6 ml vs. 17 ml +/- 6 ml) and EV (17 ml +/- 5 ml vs. 21 ml +/- 6 ml) increased significantly during therapy with UDCA (p < 0.05). EF did not change significantly. After erythromycin application RV decreased to its original values (13 ml +/- 6 ml), whereas EV (24 ml +/- 6 ml) and EF (58% +/- 9% vs. 66% +/- 11%) increased significantly (p < 0.05). Thus, administration of a motilin agonist blunts unwanted effects on gallbladder motility during litholytic therapy with UDC.