Urine output is an established measure of patient effective blood volume (EBV) and a surrogate for tissue perfusion. Urine output is typically measured at one-hour intervals and expressed in milliliters per hour (ml/h). Because small volumes are difficult to measure, initial information only becomes available 20-30 minutes after catheter insertion extrapolating to a full hour. This extrapolation can result in significant overestimation or underestimation. Lately, digital urine meters have been developed as a nursing tool to reduce urine output reading errors by up to 30%. 1 During the process of validating the performance of one model (URINFOTM, FlowSense Medical, Misgav, Israel) we realized that because the monitor measures urine volume in one-minute increments, it effectively provides a continuous measurement of urine flow minute by minute. In a previous study, we evaluated the 1-minute change in urinary flow rate (UFR) in response to hemorrhage.2 During the study we noted that during euvolemic conditions the UFR is not constant, i.e. there is minute-to-minute variability per minute. Even more surprising was the observation that this variability disappears with the onset of hypovolemia.2 (Fig. 1). The aim of this study was to describe this new physiological phenomenon (minute-to-minute variability of UFR) and its relationship to EBV depletion. Methods The Hebrew University Animal Care and Use Committee approved the protocol (MD-07-10924-2), and each study was supervised by a veterinarian. Each trial was conducted after one week of acclimatization under veterinary care. Seven adult female pigs, weighing 50±10 kg, were fasted overnight before the start of the protocol with unlimited access to water. Induction of anesthesia was achieved with an intramuscular injection…… half of the paper……. 2011;26:509-15.4. Chesnokov YV, Chizhikov VI: Continuous wavelet transformation in processing of electrocardiograms in ventricular arrhythmia. Measurement Techniques, 2004;47:417-421.5. Hurtado R, Bub G, Herzlinger D: The pelvis-kidney junction contains HCN3, a hyperpolarization-activated cation channel that triggers ureteral peristalsis. Kidney International 2010;77:500–508.6. Proctor KG, Atapattu SA, Duncan RC. Heart rate variability index in trauma patients. J Trauma 2007;63:33-43.7. Egi A, Kawamoto M, Kurita S, Yuge O. Systolic blood pressure variability reflects alterations in circulating blood volume in hemorrhagic shock in rabbits. Shock 2007;28:733-40.8. Batchinsky AI, Cooke WH, Kuusela TA, Jordan BS, Wang JJ, Cancio LC. Sympathetic nerve activity and heart rate variability during severe hemorrhagic shock in sheep. Autonic Neurosci. 2007; 30;136:43-51.