A number of field and indoor experimental procedures were performed in three typical apartment buildings located in two street canyons characterized by different urban features, during summer period 2002 in Athens. Natural, mechanical and hybrid ventilation measurements were performed based on the tracer gas method on a circular basis within the 24-h period. The indoor exchange rates are estimated using both single and multi-zone approaches based on the mass balance of two tracer gases (N2O and SF6). The experiments pointed out that, in spite of the canyon effect, appreciable ventilation rates can be obtained with natural ventilation, especially when cross ventilation with two or more windows is measured. In the presence of cross ventilation and with sufficient ambient wind speeds, natural ventilation is shown to be more effective, with regard to ACH, in comparison with hybrid. However, when only single-sided ventilation is possible or under calm conditions hybrid ventilation has a slight advantage over natural. A comparative analysis is made for the total air change rates estimated with the single and multi-zone methodologies. The multi-zone approach, in spite of its better theoretical basis, has been found more sensitive to the accuracy of the measured concentrations, especially when a single tracer is used. (C) 2004 Elsevier B.V. All rights reserved.