A photo-optical observation of gallbladder motility was made with a cholangiography on 38 mongrel dogs. The whole area, the sectional length were calculated from the series of roentgenograms, and compared the control value (100) before caerulein administration with the values (percentage of the control value) after caerulein administration. The results obtained are as follows. 1. Preparatory experiments indicated that lateral view is most suitable and intravenous administration of caerulein is more effective. Duration of X-ray discharge was decided as every 1 minute for initial 30 minutes after caerulein administration and every 5 minutes for following 60 minutes. 2. On experiments performed on the above-mentioned condition, there were two types of gallbladder motility. A typical type (12 cases): The length of fundus had most close correlation with the change of the gallbladder area, next was body and neck had less close correlation. The longitudinal length had more less close correlation than the sectional transverse length. An atypical type (8 cases): The longitudinal length had more close correlation with the change of the gallbladder area than the sectional transverse length. 3. By analysing the mean values of a typical type, the gallbladder area (y) could be expressed by the sectional length (x) as follows: neck: y=1.68x -- 68.1 body: y=1.81x-80.0 fundus: y=1.48x-45.4 longitudinal: y=1.83x-88.9