The linearity and accuracy of noninvasive ultrasonic method of measuring beat-to-beat renal blood flow was evaluated by correlation with standard electromagnetic flowmetry. Using a combined real-time ultrasonic imager and pulsed Doppler velocimeter known as a duplex scanner (DS), lumen diameter (D) and average blood velocity (V) within the imaged renal artery were recorded. Renal blood flow ( QDS ) was calculated offline using a microprocessor from the equation QDS = (pi x D2 x V)/4. This noninvasive method had previously been validated in vitro using a controlled hydraulic system which modeled steady-state flow (QT) where QDS = 0.98 QT + 7.75, SEE = +/- 13.2, r = +0.98, P less than 0.001. In three anesthetized dogs, simultaneous QDS and electromagnetic flow ( QEMF ) measurements (range 44-484 ml x min-1) were made in the proximal left renal artery. Linear regression analysis gave QDS = 0.43 QEMF + 40.5, r = 0.78, SEE = 33.8 ml x min-1, P less than 0.01; QDS = 1.2 QEMF + 2.9, r = 0.86, SEE = 20.8 ml x min-1, P less than 0.01; QDS = 0.86 QEMF + 0.2, r = 0.93, SEE = 53.4 ml x min-1, P less than 0.01. These results suggest that noninvasive QDS measurements of renal blood flow are linear and reasonably accurate compared with invasive QEMF in dogs. The method may have utility in the noninvasive measurement of beat-to-beat blood flow in human renal arteries.