The effect of cimetropium bromide, a new potent antimuscarinic compound, on caerulein-induced gall bladder emptying in 8 male volunteers was studied by real time ultrasonography. During saline infusion, caerulein (10-40 ng.kg-1.h-1) induced dose-dependent emptying of the gall bladder. There was a significant linear correlation between the dose of the peptide and the reduction in gall bladder size. A continuous infusion of cimetropium bromide (5 mg.h-1) significantly inhibited the contracting effect of caerulein on the human gall bladder, by 74% in response to the lowest dose and by 45% and 22%, respectively, to the two higher doses. The data confirm that the contracting effect of CCK-like peptides on the human gall bladder is at least partly cholinergically mediated, and they demonstrate the relaxing activity of cimetropium previously shown in animals. Provided its antispasmodic activity is also evident in disease, cimetropium should be regarded as a potentially useful agent for the treatment of biliary colic and spasm of the biliary tree.