We have developed a device to measure continuously the slow flow of fluid secreted by the fungs of foetal lambs. Typically this fluid flows along the trachea at 5–10 ml kg−1h−1 Intermittent retardation of flow is thought to facilitate prenatal lung development. The flowmeter is inserted into an extracorporeal loop of tubing inserted into the foetal trachea. A differential U-tube pressure sensor, employing an infra-red optical system to detect movement of a meniscus, is inserted into the loop. The optical detector controls a miniature, reversible peristaftic pump which is connected in paralfel with the U-tube. The pump is activated in such a way as to correct deviations in the level of the meniscus and flow measurements are thus obtained from the rate and direction of pump motor rotation. Instantaneous flow rate and integrated flow can be recorded simultaneously. The device has proven reliable over long periods of use and could be used to measure the movement of many slow-flowing physiological liquids