Dose-response curves for cholecystokinin (CCK), cerulein (CRL), and the effect of atropine on peptide-induced gallbladder evacuation, were evaluated in 13 normal subjects. Gallbladder volume was monitored by means of real-time ultrasonography. After an overnight fast, CCK was infused in six subjects iv, with increasing doses from 0.002 IDU/kg/min for 15 min, and CRL in seven subjects from 0.5 ng/kg/min for 5 min, until maximum gallbladder evacuation (greater than 70% of the fasting volume) was achieved. Forty-eight hours after the first study, CCK and CRL were readministered at the maximum contracting dose in each subject with and without a pretreatment with atropine (1 mg iv as bolus). The results showed maximum gallbladder evacuation at 0.016 IDU for CCK, and at 4 ng for CRL. Atropine significantly blunted the gallbladder response both to CCK and to CRL. It is therefore suggested that the cholinergic system is involved in the gallbladder response to CCK and to CRL.