Outdoor air intake in dwellings equipped with mechanical exhaust ventilation is studied. The goal is to control both air flows and thermal conditions in the occupied zone. The stability of the outdoor air flow of a bed room in a detached house and in a block of flats was studied by calculations. Only when the pressure difference between the outdoor and indoor air exeeds 20 Pa in the design situation, are the variations in the outdoor air flow small, if the air is taken in directly from the outside. Hence in a sheltered terrain a pressure difference of 10 Pascals is enough. The controllability of the supply air flow improves, if the air is led to the room through a special ventilating air space. This comprises an air space left under the outer wall which goes continuously round the building or a by an attic. Also a cascade-controlled exhaust air terminal device can significantly lessen the disturbance of wind. A field test was made to study whether a supply air window could be arranged in an existing apartment building. The supply air window was arranged so that a calculated length of weather strip was removed. It was found that this kind of design of the flow ways involves many uncertainties, this explaining why the desired distribution of outdoor air flows in the flats investigated was not fully achieved. NevertheIess, the outdoor air intake remained draughtless. Controlling the thermal conditions was studied in laboratory conditions by testing the draughtlessness of some outdoor air inlets marketed in Finland. The largest draughtless air flow obtained with the best outdoor air inlet was less than 6 dm3/s, when the outdoor temperature was -20 °C and the pressure difference was a maximum of 10 Pa. This air flow is only enough to satisfy the need air flow of 1.5 persons. Later checking of the measuring method seemed to give even smaller air flows. The occupants were asked about their experiences of the functioning of the outdoor air inlets as regards indoor air. It was not possible to determine clear differences between the different ways of taking in outdoor air as the number of persons asked was quite small. However, it was found that people want to open windows regardless of the way the outdoor air intake is designed. As a measure to control the outdoor air flows in a room, the right design of flow ways, a ventilating air space and constant-flow air inlets are suggested. The thermal conditions can be controlled by the right choice for the outdoor air inlet.