To test the hypothesis that aortic baroreceptors may respond not only to changes of aortic pressure but also to changes of aortic flow, sinusoidal flow waves were applied at 1 Hz to the aorta of urethan-anesthetized rats by infusion and withdrawal of 90 microliters blood with a pump while aortic blood pressure and single baroreceptor fiber activity were monitored. Averages from 20 to 30 pressure wave forms and nerve impulse histograms were established by computer for each single fiber during control (no pump), pump instroke (I), and outstroke (O). Fiber activity increased 20 +/- 3% (mean +/- SEM, n = 28 fibers) and decreased 23 +/- 4% with respect to control during I and O, respectively, whereas estimated changes of aortic flow were +/- 21%. The number of nerve impulses increased (I) and decreased (O) mainly during late systolic pressure rise did not change significantly, but the rate of late systolic pressure fall decreased 42 +/- 7 mmHg/s (I) and increased 39 +/- 11 mmHg/s (O). These results suggest that, because of the rate sensitivity of aortic baroreceptors, changes in aortic flow are reflected in changes of nerve impulse firing patterns during the blood pressure decrease in late systole, even in the absence of significant changes in systolic, diastolic, and pulse pressure.