The xenon-133 method for measuring renal blood flow in the intact rat was evaluated by direct measurement using a nonhemolyzing pump to perfuse kidneys in situ with the rat's own blood. Flows were calculated from the xenon data by means of four commonly used types of analysis: compartmental analysis using the weighted arithmetic mean (WAM), compartmental analysis using the weighted harmonic mean (WHM), stochastic analysis (SA), and initial slope analysis (ISA). WHM and SA estimate actual blood flows, whereas WAM and ISA provide only an index of mean renal flow. All results correlated well with the pumped flows (r values ranged from 0.79 to 0.98). However, the various types of analysis gave a wide range of calculated flows. This may explain some of the variation found in mean renal flow values reported in the literature. The method of choice was WHM, using only the first two compartments; the regression line between this (y) and direct measurement (x) was y = 0.98x + 0.17, r = 0.96.