Pulmonary perivascular interstitial fluid pressure (Px) was measured as a function of extravascular water accumulation (W). Px was measured directly by wick catheters and open-ended needles inserted in the interstitium near the hilus of isolated perfused dog lobes. Lobes were studied at constant transpulmonary pressure (Ptp) and vascular pressure (Pv, arterial equal to venous pressure). Px-W behavior had two distinct phases: an initial low compliance phase interpreted as perivascular filling, followed sometimes by an abrupt transition to a high compliance phase interpreted as alveolar flooding. W at transition was between 20 and 50% of the initial lung weight. Perivascular compliance during filling at a Ptp of 6 cmH2O was 0.1 g.g wet lobe wt-1.cmH2O-1, which was one-sixth that during alveolar flooding and 2.5 times that at a Ptp of 25 cmH2O. At the start of alveolar flooding, estimated alveolar interstitial fluid pressure was slightly (2 cmH2O) below alveolar pressure (PAlv) at a Ptp of 6 cmH2O but considerably belov PAlv at high lung volumes. These findings support the concept that alveolar surface tension reduces the interstitial fluid pressure below PAlv.