Abstract An interactive software to gate scintigraphic list-mode data collected from ventilation studies using a radioiosotope of an inert gas ( 81mKr) has been developed. It allows the detection of end expiration and end inspiration phases in a time activity curve. It involves six phases: (1) filtering of the curve; (2) preselection of local extrema; (3) validation of the preselection; (4) choice of the parameters for the reconstruction; (5) control of the selected cycles and display of parameters such as frequency, expiration time, breath amplitude; (6) reconstruction of images representing the distribution of ventilation during a mean respiratory cycle. This software was tested on 116 data sets without a failure. The interactive definition and control of the selected times made the algorithm useful regardless of the count rate and the frequency. It could be adapted to gate other periodic or semiperiodic physiological phenomena.