The effect of administering different doses of erythromycin on gallbladder emptying and plasma concentrations of immunoreactive motilin was investigated in healthy volunteers. Erythromycin was infused for 30 min at four different doses: 20, 50, 100, and 1000 mg/hr. Gallbladder volume was determined by ultrasound scanning every 10 min for 60 min. All doses, except 20 mg/hr, provoked a significant reduction in gallbladder volume (P < 0.01). The gallbladder emptying peak occurred after 20 min infusion. It was approximately 40-45% of basal volume and 60-70% of the emptying observed after a standard meal. At 100 mg/hr, erythromycin caused a 2.5-fold increase in plasma motilin concentration, which reached a peak after 30 min infusion. Plasma motilin peaked following maximum gallbladder emptying in all subjects. To evaluate whether cholinergic pathways were implicated in the action of erythromycin, 100 mg/hr erythromycin was infused together with 6 micrograms/kg/hr atropine. Atropine inhibited both gallbladder emptying and motilin release (P < 0.001). Infusion of 1 microgram/kg/hr somatostatin had the same inhibitory effects (P < 0.001). Our results suggest that atropine acts by inhibiting an erythromycin-activated cholinergic neural mechanism. Somatostatin could exert its inhibitory effect by blocking the release of acetylcholine from neural terminations.