The threshold and dynamics of gallbladder emptying in human subjects in response to cholecystokinin (Pancreozymin, Boots Co. Ltd) were defined by radionuclide imaging with a gamma camera. The radiopharmaceutical employed, 99mTc-HIDA, was taken up rapidly by the liver and efficiently excreted into the biliary system so that gallbladder filling was easily distinguishable from negligible background activity. Counts were recorded continuously on magnetic tape during i.v. infusion of sequentially increasing doses of cholecystokinin: Each dose level was maintained for 15 min. At later playback, the area of interest was adjusted to include only the gallbladder and to exclude radioactivity present in the gut during gallbladder emptying. In 19 normal subjects (10 male and 9 female), a threshold dose of cholecystokin was identified for gallbladder contraction: 0.010 Crick-Harper-Raper units/kg-min in 16 subjects, and 0.020 Crick-Harper-Raper units/kg-min in the remaining 3. The rate of emptying appeared smooth and linear at each dose level: Doubling the dose of cholecystokinin in every case significantly increased the emptying rate. There appeared to be no effect of increasing age on emptying more rapidly than females, but the difference was not significant. This cholescintigraphic technique would appear to offer a simple, accurate, yet noninvasive method for continuously monitoring the events during gallbladder contraction in humans.