It has been recently shown that erythromycin, a macrolide antibiotic, exhibits prokinetic properties, by enhancing gastric emptying in health and disease and by inducing gallbladder contraction. The aim of the study was to further investigate the effect of intravenous erythromycin on gallbladder motility during fasting and postprandial states. In 10 healthy male subjects gallbladder emptying was assessed by ultrasonography on three different occasions, each in a random sequence, as follows: (1) after giving 300 ml of fresh milk and infusing normal saline as placebo (postprandial emptying), (2) after infusing 200 mg of erythromycin during the fasting state, and (3) after infusing 200 mg of erythromycin along with ingestion of 300 ml of fresh milk. Infusion of erythromycin and placebo lasted 10 min. From the emptying curves, the duration of the lag phase of emptying, the ejection fraction of emptying, and the time by which maximal emptying was achieved were calculated. Infusion of erythromycin induced an immediate contraction [lag phase (+/-SD): 1.3+/-2.6 SD min] of the gallbladder by 42.1+/-22% of its initial volume. Infusion of erythromycin during the postprandial state significantly decreased the duration of the lag phase (1.3+/-3.5 min after erythromycin plus test meal versus 3.6+/-4.2 min after test meal only, P < 0.04) and significantly increased the ejection fraction (78+/-8.5% after erythromycin plus test meal versus 60.6+/-8.5% after test meal only, P < 0.0008). It is concluded that intravenously given erythromycin induces contraction of the gallbladder during the fasting state and enhances postprandial gallbladder emptying by accelerating the initiation and increasing the extent of emptying.