W e determined the effects of hyperosmolarity on lung microvascular barrier properties by means of the split-drop technique in single venular capillaries of the isolated, blood-perfused rat lung. Using isosmolar and hyperosmolar test solutions (colloid osmotic pressure = 21 cm H2O), we quantified transcapillary flux at a fixed absorptive capillary pressure, and the capillary hydraulic conductivity (Lp). Loss of barrier function was indicated in flux reversal from isosmolar absorption to hyperosmolar filtration (P < 0. 01), and by hyperosmolarity-induced Lp increase (P < 0.01). Barrier recovery after a 1-min hyperosmolar exposure was delayed > 25 min. The flux reversal was blocked by the tyrosine kinase inhibitors genistein and MDC (P < 0.01). Genistein also inhibited the Lp increase (P < 0.01). Immunoblots of hyperosmolarity-exposed, cultured rat lung microvascular endothelial cells (RLMEC) and of endothelial cells freshly harvested from lungs given hyperosmolar infusions indicated a genistein-inhibitable enhancement of protein tyrosine phosphorylation. Immunoprecipitation studies indicated tyrosine phosphorylation of the mitogen activated protein kinases (MAPK) ERK1 and ERK2 and the adaptor protein Shc in lysates of RLMEC exposed to hyperosmolar conditions. We conclude that in lung venular capillaries hyperosmolarity deteriorates barrier properties, possibly by inducing tyrosine phosphorylation of endothelial proteins.