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Oxygen tension (Po₂) of urine in the bladder could be used to monitor risk of acute kidney injury if it varies with medullary Po₂. Therefore, we examined this relationship and characterized oxygen diffusion across walls of the ureter and bladder in anesthetized rabbits. A computational model was then developed to predict medullary Po₂ from bladder urine Po₂. Both intravenous infusion of [Phe²,Ile³,Orn⁸]-vasopressin and infusion of N^G-nitro-l-arginine reduced urinary Po₂ and medullary Po₂ (8–17%), yet had opposite effects on renal blood flow and urine flow. Changes in bladder urine Po₂ during these stimuli correlated strongly with changes in medullary Po₂ (within-rabbit r² = 0.87–0.90). Differences in the Po₂ of saline infused into the ureter close to the kidney could be detected in the bladder, although this was diminished at lesser ureteric flow. Diffusion of oxygen across the wall of the bladder was very slow, so …