We compared pulmonary capillary filtration coefficients (Kf,c) using measurements of transcapillary filtration rates based on laser densitometry of perfusate hematocrit changes (Jy,l) and gravimetric measurement of the rate of lobe weight gain (delta Wt/delta t) after an increase in capillary pressure (Pc) in isolated autologous blood-perfused dog lungs. Although the lobe weight increased at a rate that decreased with time, the densitometric filtration rate was relatively constant over time. Kf,c values were calculated in milliliters per minute per centimeter water per 100 g from filtration rates obtained by 1) extrapolation of the delta Wt/delta t from 3 to 10 min back to time 0 [Kf,c(0)], 2) use of the slope of the line fitted to the delta Wt/delta t and Jv,l values obtained after three stepwise increases in Pc [Kf,c(slope)], and 3) use of delta Wt/delta t and Jv,l values obtained 10, 20, and 30 min after either a constant 30-min or three 10-min stepwise increases in Pc [Kf,c(t)]. The mean Kf,c(0) values ranged from 0.171 +/- 0.024 to 0.188 +/- 0.070 and were not significantly different between vascular pressure states. These Kf,c(0) values were significantly higher than all gravimetric or densitometric Kf,c(t) or Kf,c(slope) values that ranged from 0.037 to 0.100. Kf,c(t) values were not statistically different from each other by use of either method, but the gravimetric Kf,c(slope) was significantly higher than the densitometric value. Therefore, extrapolated Kf,c(0) probably overestimates endothelial barrier Kf,c because of persistent vascular stress relaxation after a vascular pressure increase. Adjustment of tissue Starling forces may occur rapidly after an increase in filtration pressure.