Background/Aims: The validity of fluorescence optodes for measurement of renal cortical tissue oxygen tension was tested by comparison with Clark electrodes. Methods: We varied renal blood flow and inspired O<sub>2</sub> content in anaesthetized rabbits while simultaneously measuring cortical tissue oxygen tension. Results: Cortical oxygen tension varied with inspired O<sub>2</sub> content. Fluorescence optode measurements were more tightly distributed than those from a Clark electrode. Cumulative frequency distributions for fluorescence optodes were shifted to the left of those for Clark electrodes. The slope of the relationship between oxygen tension in arterial blood and cortical tissue was less for the fluorescence optode than the Clark electrode. Cortical tissue oxygen tension measurements by these two methods were correlated (r<sup>2</sup> = 0.32; p < 0.001), with no fixed bias but considerable proportional bias. Thus, the slope of the relationship between the two measurements was less than unity (0.57 [0.50–0.69]). Conclusion: Cortical oxygen tension values from fluorescence optodes are less variable but proportionally less than those from Clark electrodes. Theoretical considerations suggest that true interstitial oxygen tension lies somewhere between values provided by the two techniques. Nevertheless, the lesser variability of the fluorescence optode technique may aid detection of physiologically significant changes in intrarenal oxygenation.