We evaluated whether tumor necrosis factor (TNF)- induces an increase in permeability of an alveolar epithelial monolayer via gelatinase secretion and basement membrane degradation. Gelatinase secretion and epithelial permeability to radiolabeled albumin under unstimulated and TNF--stimulated conditions of an A549 human epithelial cell line were evaluated in vitro. TNF- induced both upregulation of a 92-kDa gelatinolytic activity (pro form in cell supernatant and activated form in extracellular matrix) and an increase in the epithelial permeability coefficient compared with the unstimulated condition (control: 1.34 ± 0.04 × 10⁶ cm/s; 1 µg/ml TNF-: 1.47 ± 0.05 × 10⁶ cm/s, P < 0.05). The permeability increase in the TNF--stimulated condition involved both paracellular permeability, with gap formation visualized by actin cytoskeleton staining, and basement membrane permeability, with an increase in the basement membrane permeability coefficient (determined after cell removal; control: 2.58 ± 0.07 × 10⁶ cm/s; 1 µg/ml TNF-: 2.82 ± 0.02.10⁶ × cm/s, P < 0.05). Because addition of gelatinase inhibitors [tissue inhibitor of metalloproteinase (TIMP)-1 or BB-3103] to cell supernatants failed to inhibit the permeability increase, the gelatinase-inhibitor balance in the cellular microenvironment was further evaluated by cell culture on a radiolabeled collagen matrix. In the unstimulated condition, spontaneous collagenolytic activity inhibited by addition to the matrix of 1 µg/ml TIMP-1 or 10⁶ M BB-3103 was found. TNF- failed to increase this collagenolytic activity because it was associated with dose-dependent upregulation of TIMP-1 secretion by alveolar epithelial cells. In conclusion, induction by TNF- of upregulation of both the 92-kDa gelatinase and its inhibitor TIMP-1 results in maintenance of the gelatinase-inhibitor balance, indicating that basement membrane degradation does not mediate the TNF--induced increase in alveolar epithelial monolayer permeability.