PURPOSE: To assess the effect of acute changes in renal vascular resistance (RVR) and pulse pressure on the resistive index (RI) measured by using Doppler ultrasonography (US). MATERIALS AND METHODS: Rabbit kidneys were perfused by using a pulsatile perfusion system in which RVR, systolic and diastolic pulse pressures, and pulse kinetics were controlled and monitored while simultaneously measuring the RI. RESULTS: When RVR was increased fivefold with phenylephrine hydrochloride, the RI increased only slightly (from 0.45 at baseline up to 0.50). There was a virtually linear relationship between the RI and the pulse pressure index ([systolic pressure - diastolic pressure]/systolic pressure) in the range of 0.30-0.80. The RI was not affected by the pulse rate or fraction of time that systolic pressure was applied during the pulse cycle. CONCLUSION: Contrary to conventional teaching, which is based on theoretic considerations, the RI is not readily affected by acute changes in RVR. This indicates a need to reconsider the conventional explanations used to explain increases in RI that are frequently found in patients with renal disease or ureteral obstruction.