Ultrasonography is the accepted method of choice for registering foetal breathing movements in man. The conventional apparatus hitherto used for this purpose is disadvantageous and could yield misleading artefacts. A new differential echoscope has been designed and developed especially for recording intrauterine foetal movements. It allows a simultaneous registration of movements of two foetal structures: the proximal and distal thoracic wall, or the foetal heart and the thoracic wall. It is equipped with two alternately emitting transducers, each with a repetition frequency of 150 HZ, and an intensity-modulated display of the triggering echoes within the gates. Time-to-volgate converters transform the movements of two gated echoes into electrical signals. An internal differential circuit produces on-line recordings of the true difference between movements of the two selected echoes. This device will appreciably facilitate the selection of proper echoes and reduce signal artefacts during foetal monitoring. Various errors and pitfalls in measuring foetal breathing movements are discussed.