The effects of short periods of high pulmonary vascular pressures on the filtration characteristics of the lung's microcirculation have been studied in isolated dog lungs. When left atrial pressures (Pla) were less than 30 cmH2O, the capillary filtration coefficient (Kf,c) was not different from that of controls (0.192 +/- 0.009 ml X min-1 X cmH2O-1 X 100 g-1). In 17 of 23 observations, where Pla exceeded 55 cmH2O, Kf,c was at least 1.5 times control. In 10 of 13 lungs, Kf,c returned to control values within 2 h after the Pla increase. However, in the other three experiments, Kf,c did not return to control values. These findings indicate that the filtration coefficient of pulmonary microvessels increases at high left atrial pressures (greater than 55 cmH2O) and may represent either a "stretching" of existing pores or opening of a few large pores.