While machine perfusion of explanted kidneys is theoretically superior to standard cold storage, it may damage potential transplants unless machine-associated swelling is controlled. This thesis presents the effects of perfusate tonicity on renal swelling during hypothermic machine perfusion. Phosphate buffered solution (PBS) and PBS supplemented with 5% w/v mannitol were used as isotonic (289 mOsm/kg) and hypertonic (568 mOsm/kg) perfusates, respectively. Porcine kidney pairs were procured then flushed and machine perfused; the right and left kidneys were assigned opposite perfusates. An experimental methodology was developed to image porcine kidneys undergoing hypothermic machine perfusion (5 deg C) for 15 minutes followed by 120 minutes without perfusion to quantify surface displacement (renal swelling) with digital image correlation (DIC). Surface displacement and size (thickness) were compared between the right and left kidneys of each pair. In addition, discharged renal fluids (i.e., filtrate and venous outflow) and biopsies were collected. On average, kidneys perfused with the mannitol solution were smaller in size than the kidneys perfused with PBS (p < 0.05) at the start and end of each experiment; however, there was no significant difference between the renal sizes at the end of the 15 minute perfusion interval (p > 0.05). Thus, hypertonic and isotonic perfusates yielded different renal swelling outcomes (i.e., physical size and surface displacement), which suggests that perfusate tonicity influences renal swelling. These experiments are the first time ex vivo renal surface displacement measurements have been collected during machine perfusion. / Master of Science / There is a chronic shortage of transplantable organs in the United States. As more marginal donor organs are being used to increase the number of transplantable organs, there has been a renewed interest in machine perfusion for organ preservation. While machine perfusion of explanted kidneys is theoretically superior to standard cold storage, it may damage potential transplants unless machine-associated swelling is controlled. This thesis presents the effects of perfusate tonicity on renal swelling during hypothermic machine perfusion. Isotonic and hypertonic solutions were used as perfusates. Porcine kidney pairs were procured then flushed and machine perfused; the right and left kidneys were assigned opposite perfusates. An experimental methodology was developed to image porcine kidneys undergoing hypothermic machine perfusion (5 °C) for 15 minutes followed by 120 minutes without perfusion to quantify surface displacement (renal swelling) with digital image correlation (DIC). DIC is a non-contact technique for measuring shape and displacement in three-dimensions. The results from these experiments show that hypertonic and isotonic fluids yielded different renal swelling outcomes (i.e., physical size and surface displacement), which suggests that tonicity influences renal swelling. This indicates that perfusate tonicity is an important factor for improving renal preservation with machine perfusion, which may increase the number of organs available for transplantation.