Reports measurements of air change rates made in the tower of an eleven-storey building using sulphur hexafluoride as a tracer gas. Inside to outside pressure differences were also monitored as a function of temperature and wind speed. Gives expression for autumn and winter air change rate as a function of windspeed. Reports finding that wind direction and stack effect had little effect on the air change rate. In this building toilet exhausts and other weather independent mechanisms were more important than natural infiltration.