Although it is generally assumed that release of cholecystokinin (CCK) is the chief mechanism by which a fatty meal causes contraction of the gallbladder, measured release of CCK and gallbladder contraction have never been correlated. We have achieved this correlation in eight adult male volunteers, by means of a specific radioimmunoassay for CCK and by ultrasonographic imaging of the gallbladder. This study validates our CCK radioimmunoassay and correlates measured concentrations of CCK with changes in gallbladder size measured by ultrasonographic examination. Basal concentrations of CCK (82.6 +/- 10.4 pg/ml) rose significantly to a maximum of 411.1 +/- 79.9 pg/ml at 16 minutes after intraduodenal instillation of medium-chain triglyceride (Lipomul). Mean basal volume of the gallbladder was 34.6 cm3; maximum reduction of gallbladder volume (to one-third of original) was achieved at 18 minutes. Elevated CCK concentrations began to fall toward basal, and the gallbladder began to refill at 25 minutes. Results obtained after oral ingestion of Lipomul provide similar results. Linear regression analysis demonstrated excellent correlation between concentrations of CCK and gallbladder size during both contraction and relaxation phases. Future study of this correlation may be useful in patients with manifest dysfunction of the gallbladder, as well as in individuals known to be at risk of gallbladder disease.