The postprandial contractions of the gall bladder result from the interaction of neurohormonal factors but their relative contribution is unknown. This study was designed to determine the role of cholecystokinin (CCK) in gall bladder contractions using a highly selective and potent CCK-receptor antagonist, CR-1505 (loxiglumide) in healthy men either infused with exogenous CCK in graded doses (1.56-50 pmol/kg/h) or subjected to modified sham feeding (MSF) and ordinary feeding tests. The gall bladder volume measured by real time ultrasonography showed dose dependent decrease in the gall bladder volume in 10 subjects when CCK8 was infused iv in graded doses reaching about 15% at 1.56 pmol/kg/h and 91% at 50 pmol/kg/h. Close correlation between the decrease in gall bladder volume and the dosage of CCK or the increments in plasma CCK-bioactivity was observed. After pretreatment with loxiglumide, CCK resulted in similar increments in plasma CCK-bioactivity but failed to affect the gall bladder volume at CCK doses up to 6.25 pmol/kg/h and caused only 53% reduction at 50 pmol/kg/h. Modified sham feeding and real feeding reduced the volume of gall bladder by 20% and 70%, respectively and loxiglumide decreased these values to 15% and 30%, respectively. This study provides evidence that loxiglumide is highly potent and selective CCK antagonist and that endogenous CCK plays an important role both in the postprandial contractions of gall bladder.