In ischemic organs, the protein reflection coefficient () can be estimated by measuring blood hematocrit (Hct) and protein after increasing static vascular pressure (P_v). Our original equation for  (J Appl Physiol 73: 2616-2622, 1992) assumed a constant vascular volume during convective fluid flux (). In this study, we 1) quantified the rate of vascular volume change (dV/dt) still present in ischemic single ferret lungs after 20 min of P_v = 30 Torr and 2) developed an equation for  that allowed a finite dV/dt. In 25 lungs, we estimated the dV/dt after 20 min at P_v = 30 Torr by subtracting from the rate of lung weight gain (_L). The relationship between (0.15 ± 0.02 ml/min) and _L (0.24 ± 0.02 g/min) was significant (R = 0.66, P < 0.001), but the slope was <1 (0.41 ± 0.10, P < 0.05). dV/dt (0.10 ± 0.02 ml/min) was similar in magnitude to at 20 min. The modified equation for  revealed that a finite dV/dt caused the original  measurement to underestimate true . A low , high , high baseline Hct, and long filtration time enhanced the error. The error was small, however, and could be minimized by adjusting experimental parameters.