The aim of this study was to estimate the specific arterial water space and associated blood flow using statistical moments of the frequency versus time outflow profile, with a model with specific spaces for hepatic arterial (HA) and portal venous (PV) flows in parallel with a common space. Studies were performed in the in situ dual-perfused rat liver (n = 6-10), using Krebs-bicarbonate buffer with constant PV flow (12 ml/min) and various HA flow rates (3-6 ml/min). An impulse input-output technique was employed, varying the route of input, using [¹⁴C]urea as the reference indicator. Regardless of flow conditions, the frequency outflow profile after HA input was flatter and broader and the mean transit time longer than after PV input. Excellent recovery of marker was obtained in all cases. Applying the above model, the specific arterial space was estimated to be 9.7 ± 2.3 of total water space and receives ~17% of the HA flow, with the remainder mixing with portal blood in the common space. The estimated total water content of liver (0.67-0.72 ml/g liver) agrees well with that determined by desiccation (0.72 ± 0.01 ml/g liver).