SUMMARY  Rb‐86 and Cs‐134 were given intravenously to dogs whilst a scintillation counter was placed over the exposed kidney. The kidney took up its maximum of Rb‐86 within two minutes of injection. The corresponding peak occurred from fifteen to twenty minutes after injection for Cs‐134.  The content of Rb‐86 and Cs‐134 in kidney, liver, spleen, muscle, and blood, was measured at intervals up to thirty minutes after intravenous injection. Of these tissues none was found to contain more than a fraction of the concentration of Rb‐86 in the kidney at the end of the first two minutes. If the injected Rb‐86 were contaminated with Cs‐134, a significant masking of the renal uptake of Rb‐86 would occur, and the precision of the method would be correspondingly impaired.  The renal uptake of Rb‐86 is sufficiently rapid for it to depend on the arterial blood supply, and because the renal concentration is much higher than that of surrounding tissues its measurement by external counting techniques is possible.  External scintillation counting of the renal uptake of cæsium‐free Rb‐86 appears to be a valid method for comparing the blood flow of the two kidneys.