Our novel transit-time blood volume flowmeter will be validated in vivo, by long-term implantation on the left carotid artery of a sheep. Permanent cannulation of this artery will permit daily flowmeter calibration. Equipment for transcutaneous blood volume flow measurement will be completed and tested. Amongst these are: a dual transmit-receive ultrasonic probe system; a multichannel pulsed Doppler detection system; a digitised average Doppler frequency calculator with angle of illumination correction; circuitry to resolve the artery inside diameter to 0.1 mm accuracy. The radiated soundfield of stripshaped piezo-electric ceramics will be measured as a function of frequency, transducer material and geometry, and backing and matching conditions.