The filtration variables, K (filtration coefficient), Ppmv (perimicrovascular pressure) and sigma (reflection coefficient), were estimated independently in previous reports using the Starling equation or the micropuncture method. We used matrix algebra to estimate these variables simultaneously. We measured filtration rate (Q) by a gravimetric method in isolated rat lung lobes in zone 1 conditions (alveolar pressure = 20 cmH2O) at two vascular pressures, Pvasc = 15 or 18 cm H2O and perfused the lobes with plasma containing a low or a high concentration of protein. By extrapolating the log of the rate of weight gain to t = 0, we obtain the initial filtration rate before any of the pressure variables (microvascular and perimicrovascular hydrostatic pressures) in the Starling equation changed. Assuming that protein filtered into perimicrovascular space only by convection, we substituted it into the Starling equation as follows: Q = K [(Pmv -- Ppmv) -- sigma 2 (IImv)], where Pmv and IImv are microvascular and perimicrovascular plasma protein osmotic pressures. IImv was estimated by Yamada's equation (Yamada et al. 1985). For the matrix algebra, we used three values, we omitted the value for the high protein, low vascular pressure experiment. We obtained K = 26.3 [mg/(min x cmH2O x g wet weight)], Ppmv = 6.2 cmH2O and sigma = 0.46. These values agree with values from previous reports. Since these 3 filtration variables are interrelated, this new method for simultaneous measurement is more accurate than independent measurements are. The chief advantage of this method is that it does not require a separate estimate of isogravimetric pressure or a direct measurement of interstitial pressure, and all variables are obtained simultaneously.