We have carried out simultaneous, quantitative measurements of absolute gallbladder (GB) storage and emptying in 6 subjects with gallstones, using a modified duodenal perfusion technique that incorporates technetium 99m-labeled diethyl phenylcarbamomethyl iminodiacetate (99mTc-HIDA) as a GB bile marker in addition to indocyanine green as a hepatic bile marker. The technique was validated by measuring duodenal recovery of 99mTc-HIDA (mean +/- SEM, 101% +/- 4%), and also by studying 2 subjects who had undergone cholecystectomy. During the first hour following an evening meal, cumulative GB ejection of 99mTc-HIDA in the 6 subjects with gallstones was 43% +/- 12%. This was accompanied by GB storage of most hepatic indocyanine green (70% +/- 5%) during each 10-min interval of that hour. During subsequent overnight fasting, storage of hepatic indocyanine green (89% +/- 2%) was accompanied by ejection of 99mTc-HIDA from the GB in 33 of the 66 hourly intervals. Since 'simultaneous' filling and emptying cannot occur through the cystic duct, the results suggest frequent alternations in absolute GB storage and emptying, a phenomenon more analogous to a bellows than to the conventional concept of a simple pump.