The filtration variables, filtration coefficient (K), perimicrovascular pressure (Ppmv) and reflection coefficient (sigma) were estimated independently in previous reports using the Starling Equation or the micropuncture method. We estimated these variables simultaneously. We measured filtration rate by a gravimetric method in isolated rat lung lobes in zone 1 conditions (alveolar pressure = 20 cmH2O) at two vascular pressures, Pvasc = 15 or 8 cmH2O and perfused the lobes with plasma containing a low or high concentration of protein. By extrapolating the log of the rate of weight gain to time = 0, we obtain the initial filtration rate. Assuming that protein filtered into perimicrovascular space only by convection, we substituted into the Starling Equation as follows: Q = K[(Pmv-Ppmv)-sigma 2 pi mv], where Pmv and pi mv are hydrostatic and plasma protein osmotic pressures in microvascular space. pi mv was estimated by Yamada's equation. We obtained K = 26.3 +/- 8.7 mg/(min.cmH2O.g), PPMV = 6.2 +/- 0.7 cmH2O, sigma = 0.46 +/- 0.07. The chief advantages of this method are that it does not require a separate estimation of isogravimetric pressure or a direct measurement of interstitial pressure, and that all variables are obtained simultaneously.