Impaired gallbladder motility and delayed intestinal transit contribute to cholesterol gallstone formation by impeding the enterohepatic circulation of bile salts and causing gallbladder stasis. The therapeutic value of erythromycin, a prokinetic motilin analog, was evaluated in an animal model of gallstone formation. Eighty ground squirrels were fed either a trace- (control) or a high- (1%) cholesterol diet. Half of each diet group received either erythromycin stearate or placebo orally twice daily for 4 weeks. Biliary lipid secretion and bile salt pool size were determined via common duct cannulation. Gallbladder contractile response to cholecystokinin (CCK) was studied in vitro. Intestinal transit was evaluated in vivo by 51Cr marker. In the placebo-treated group, fed the high- versus the trace-cholesterol diet, bile salt secretion decreased (trace-cholesterol + placebo, 21.0 +/- 1.8 nmol/min/g liver vs. high-cholesterol + placebo, 9.3 +/- 1.4 nmol/min/g liver), cholesterol saturation index (CSI) doubled (trace-cholesterol + placebo, 0.61 +/- 0.06 vs. high-cholesterol + placebo, 1.30 +/- 0.04), nucleation time shortened (trace-cholesterol + placebo, > 21 days vs. high-cholesterol + placebo, 6.4 +/- 1.0 days), cholesterol crystals formed, gallbladder contractility diminished, and intestinal transit was delayed (each P < .05). Erythromycin treatment of animals on the high-cholesterol diet restored gallbladder contractility and intestinal transit to control levels, increased bile salt secretion, reduced the total bile salt pool, lowered the cholesterol saturation of bile, lengthened the nucleation time, and so reduced crystal formation (each P < .05). Erythromycin enhances gallbladder motility and hastens intestinal transit, promoting more rapid enterohepatic cycling of bile salts. This increases bile salt secretion, improves cholesterol solubility, and reduces crystal development.