Urine flow is a time-honored measure of the patient's effective blood volume (EBV) and a surrogate for tissue perfusion. Urine output is usually measured at hourly intervals and expressed in milliliters per hour (ml/h). Because small volumes are difficult to measure, initial information is only available 20 to 30 minutes after catheter insertion, extrapolating to a full hour. This extrapolation can result in a considerable overestimation or underestimation. Recently, 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 a model (URINFOTM, FlowSense Medical, Misgav, Israel), we realized that since the monitor measures urine volume in one-minute increments, it actually provides a continuous, real-time measurement of urine flow, minute by minute. In a previous study, we evaluated the change in urine flow rate (UFR) in 1 minute in response to hemorrhage.2 During the study, we noticed that under euvolemic conditions, the UFR is not constant , that is, there is minute-to-minute variability. Even more striking was the observation that this variability disappears as hypovolemia ensues2 (Fig. 1). The aim of this study was to describe this new physiological phenomenon (minute-to-minute variability of UFR) and its relationship with 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 test was carried out after one week of acclimation under veterinary care. Seven adult sows, weighing 50 ± 10 kg, were fasted overnight before the start of the protocol with unlimited access to water. Induction of anesthesia was performed by intramuscular injection...... middle of article....... 2011;26:509-15.4. Chesnokov YV, Chizhikov VI: Continuous wavelet transform in the 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. Autonomous neurosciences. 2007; 30;136:43-51.