Abstract A flow-meter unit operating on the Venturi principle has been devised and constructed for recording of instantaneous ventilatory flow. The device, which is designed for unidirectional flow, is primarily dimensioned for measurements on humans and permits a single breathe-through system to cope with flow rates ranging from those during resting or subnormal ventilation to those occurring during heavy exercise. The pressure difference obtaining between the base and throat of the meter and used for measuring the flow exceeds the overall pressure loss by a factor of 5 or more, permitting the use of a standard differential pressure gage for accurate translation of flow into an electronic signal. Overall resistance to flow increases linearly and, with a throat diameter of 11.1 mm, amounts to 0.45 cm H 2O/1/sec at a flow of 3.3 1/sec. The unit incorporates electronic square root circuitry to obtain an output voltage that varies linearly with the instantaneous volume rate of flow. Linearity is within 1 per cent of the rated range of the meter. The unit is suitable for use with conventional electronic integrators for analog computation of tidal volumes.