Oral Emtobil, a liquid preparation of sorbitol and peanut oil, has been used in roentgenological practice for several years. Emptying of an opacified gallbladder is seen within 15 min after intake of 100 ml of this solution. The main physiological factor responsible for contraction of the gallbladder is cholecystokinin (CCK). In the present study plasma concentrations of CCK in fasting, healthy subjects were studied in response to oral Emtobil. The radioimmunoassay used measures sulphated CCK-8 and CCK-33 with equimolar potency. Neither non-sulphated CCK nor any gastrins are measured. The average concentration, after an overnight fast in nine healthy subjects without gallstones, was 1.2 pmol/l. A peak concentration was seen within 30 min after 'the test meal' and averaged 8 pmol/l. It is suggested that oral Emtobil contracts the gallbladder by release of CCK. Since Emtobil induces a fast and marked rise in the plasma concentration of CCK, it may be used in test procedures to estimate the secretion of CCK.