(1) Intravenous injection of ioxaglate (4 g iodine kg(-1)), an iodinated radiographic contrast medium, caused a marked protein extravasation, pulmonary oedema and a decrease in the arterial partial oxygen pressure in rats. (2) All of these reactions to ioxaglate were reversed by the pretreatment with gabexate mesilate (10 and 50 mg kg(-1), 5 min prior to injection) or nafamostat mesilate (3 and 10 mg kg(-1)), in which the inhibition was complete after injection of nafamostat mesilate (10 mg kg(-1)). (3) Both gabexate mesilate and nafamostat mesilate inhibited the activity of purified human lung tryptase, although the latter compound was far more potent than the former. (4) Ioxaglate enhanced the nafamostat-sensitive protease activity in the extracellular fluid of rat peritoneal mast cell suspensions. (5) Tryptase enhanced the permeability of protein through the monolayer of cultured human pulmonary arterial endothelial cells. Ioxaglate, when applied in combination with rat peritoneal mast cells, also produced the endothelial barrier dysfunction. These effects of tryptase and ioxaglate were reversed by nafamostat mesilate. (6) Consistent with these findings, immunofluorescence morphological analysis revealed that tryptase or ioxaglate in combination with mast cells increased actin stress fibre formation while decreasing VE-cadherin immunoreactivity. Both of these actions of tryptase and ioxaglate were reversed by nafamostat mesilate. (7) These findings suggest that tryptase liberated from mast cells plays a crucial role in the ioxaglate-induced pulmonary dysfunction. In this respect, nafamostat mesilate may become a useful agent for the cure or prevention of severe adverse reactions to radiographic contrast media.