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We compared simultaneous noninvasive and invasive determinations of blood velocity and flow in the renal arteries of 5 mongrel dogs. Noninvasive measurements of blood velocity spectra (Â¿FN) were made using an ultrasonic echo-Doppler duplex scanner. Vessel diameters (DN) and Doppler angles (Â¿N) were measured from sector images of the renal artery. Invasive measurements of blood velocity spectra (Â¿F1) were made using a catheter Doppler velocimeter. Vessel lumen diameter (D1) and Doppler angle (Â¿1) were measured angiographically. Using the Doppler and continuity equations, temporal mean blood velocities (VN, V1) and flows (QN, Q1) of five cardiac cycles were calculated. The ranges of velocity (8-48 cm/s) and flow (40-380 ml/min) were varied by pharmacological intervention. Standard linear regressions (n = 33) were Results suggest that simultaneous noninvasive echo-Doppler and invasive catheter Doppler measurements of canine renal artery blood velocity and flow correlate significantly. Nevertheless, large standard errors of the estimates exist which suggest that important systematic and experimental errors are present in both methods.