Renal ischemia reperfusion injury (IRI) is a major cause of acute kidney injury (AKI), which can lead to a rapid (within 48h) reduction in the kidney function¹. The kidneys main function is to maintain body water/salt balance and to secrete waster products produced in the body. To preserve this function, a medullary osmotic gradient is needed to maintain the ability to regulate the concentration of final excreted urine. Renal tubules are the main site of water-salt exchange, and here dysregulation of essential transport proteins like aquaporins, sodium, and urea transporters is seen in response to IRI². IRI is often diagnosed by measuring blood urea nitrogen (BUN) and creatinine clearance. Albeit sensitive, the residual function of the contralateral kidney (in unilateral disease) reduces the specificity of these methods. Thus, a particularly interesting alternative in renal investigations is hyperpolarized non-metabolic bio probes³. A particularly appealing bio-probe for renal investigations is urea. We therefore suggest that monitoring of the renal functional filtrations state via the renal cortico-medullary urea gradient may contribute to a more detailed and sensitive quantitative measure of renal status in patients experiencing AKI.Twelve female Wistar Rats were included in the study. Rats were subjected to unilateral renal ischemia for 60 min followed by reperfusion for 24 hours, the contralateral healthy kidney served as control. A tail vein catheter was inserted for injection of hyperpolarized [¹³C,¹⁵N₂]urea before the MRI scanning. A ²³Na MR examination was performed with a gradient echo sequence, (TR/TE/flip angle/ spectral width/ matrix / FOV) 50 ms/2.0 ms/90°/10 kHz/ 32 x 32 x 8 / 60 x 60 x 60, hereafter a 3D fully balanced steady state imaging sequence with hard pulse excitation and refocusing pulses was employed for 3D imaging of the kidney ¹³C,¹⁵N urea distribution. The experimental parameters where (TR/TE/flip angle/spectral width/matrix/FOV) 5.4 ms/2.7 ms/15°/20 kHz/48 x 48 x 48/60 x 60 x 60. The experiments were performed in a pre-clinical 9.4T MRI (Agilent) equipped with a triple-tuned Doety ¹H/¹³C/²³Na coil. After MRI scanning, kidneys were excised and stored for further biochemical analyses.All rats in this study showed evidence of severe damage to the IRI kidney 24 hours after the surgery (Figure 1). The total kidney ¹³C urea signal intensity was significantly decreased with 56% in the IRI kidney compared to the control kidney. This was associated with a decreased urea cortico-medullary gradient of 39% in the outer medulla (OM) and 70% in the inner medulla (IM) (Figure 2). This correlated with similar decreases in sodium gradient (19% in both OM and IM), which has previously been shown also to correlate with IRI⁴. A general down regulation of urea transporters UT-A1, UT-A3 and UT-B was evident and correlates with several studies indicating a downregulation of urea transporters associated with the loss of cortico-medullary ion gradient^5,6The fact that the intra-renal urea distribution is altered and secondly observable in the injured kidney, demonstrate the potential of hyperpolarized urea examinations in the diseased kidney in human subjects. The recent translation of hyperpolarized MRI to human patients⁷ and the introduction of high resolution mapping of the intra-renal distribution of urea⁸, may provide valuable clinical single kidney functional information not obtainable by other means