This paper provides some results toward establishing the relationship between the initial infiltration, natural ventilation and HVAC ventilation performance. The air exchange rate measurements were performed for the building leakage, the combination of window positions & ventilator options, and the HVAC system condition given different air flow rates in the Intelligent Workplace. But, the air exchange rates cannot be directly compared to each other because they were measured at different times having different environmental conditions. As a first attempt to understand the relative influence of the parameters involved, a simplified relationship was derived based on the assumption that the air change rate is proportional to the square root of the indoor-outdoor temperature difference, to outdoor air velocity, and to the geometric leakage area. These considerations and the available measurement data suggested the formulation. There was relatively good agreement between predicted and measured values for the test bay based on this relationship.