Reports on a measurement system developed by the Institute of Technology at Tastrup Denmark, involving a microcomputer- controlled system for registering air change rates using tracer gas (nitrous oxide) according to the constant concentration method. The system is designed for measuring and metering tracer gas in up to 10 separate rooms (using an infrared gas absorption detector). It operates through automatic data logging on a floppy disc and can run without supervision for extended periods (up to six days). This means that the building's true air change rate over a long period can be measured, taking into account factors such as occupancy effects. Gives the results of one day's measurements in a house, and describes the difficulties encountered in maintaining a constant concentration of tracer gas.