We have studied the effect of atropine on cholecystokinin (CCK)-induced gallbladder contraction in 7 healthy volunteers by means of real-time ultrasonography. Two series of tests were carried out in random order and on separate days. In one series of tests, CCK alone was infused for 4 successive 15-min periods at sequentially increasing doses of 0.0021, 0.0042, 0.0084, and 0.0168 Ivy dog units (IDU) X kg-1 X min-1. In the other series of tests, an infusion of a low dose of atropine, 5 micrograms X kg-1 X h-1, was added to the hormone infusion. The smallest dose of CCK which significantly contracted the gallbladder was 0.0042 IDU X kg-1 X min-1. The highest dose of CCK infused, 0.0168 IDU X kg-1 X min-1, produced almost total contraction of the organ. In all subjects, the infusion of atropine completely blocked the gallbladder response to 0.0042 and 0.0084 IDU X kg-1 X min-1, and partially inhibited (by 52%) the response to the highest dose. In 2 subjects in whom a higher dose of atropine, 15 micrograms X kg-1 X h-1, was tested, gallbladder contraction was totally abolished, even when the largest dose of CCK was infused. Contrary to what is generally believed, the results indicate that the response of human gallbladder to CCK is largely dependent on cholinergic innervation.