We studied 12 normal subjects who underwent four 99Tcm-HIDA examinations. Imaging was performed with 111 MBq at 1 image/min for 60 min, and was registered on a computer on a 64 x 64 word matrix. Normalized and background corrected time-activity curves on the gallbladder were obtained from which total and cumulative 10 min interval emptying was calculated. During the first examination (control), 10 min after the beginning of acquisition, 1 ml saline solution was injected subcutaneously. The second examination was performed injecting 5 mg bethanechol subcutaneously instead of saline solution. During the third and fourth examinations 10 mg pirenzepine and 0.15 mg/10 kg atropine were injected i.v. respectively, 5 min before bethanechol injection. Wilcoxon's test was used to compare the first and second studies and the latter with the third and fourth studies. The median value of gallbladder emptying at 60 min was 2% with saline solution injection and 27.5% with bethanechol. This difference was significant from minute 30 onwards. Atropine administration inhibited gallbladder emptying completely in all cases, with significant differences (P less than 0.01) in relation to bethanechol values following the first 30 min of the examination. Gallbladder emptying was observed in the study with pirenzepine, but was significantly inferior to the bethanechol values after the first 30 min.