Several epidemiological studies suggest that exposure to house dust mite allergens plays a role in the pathogenesis of asthma. Since many of these allergens exhibit enzymatic properties, they may damage the airway epithelium. To characterize the effects of low doses of Dermatophagoides pteronyssinus on the airway epithelium, the effect of D. pteronyssinus on the epithelial bioelectric properties of tracheal fragments of non-sensitized Lewis rats was studied, using Ussing-type chamber technique. The addition of a crude D. pteronyssinus extract containing 20 microg mL(-1) of Der pI allergen in the presence of 1.5 mM dithiothreitol (DTT, an activator of cysteine proteases), induced a progressive increase in bioelectrical conductance (+12.0+/-1.5%, n=12, p<0.005), an index of epithelial permeability, without affecting the short circuit current (which reflects active ion transports). The D. pteronyssinus-induced increase in epithelial conductance was related to the cysteine-protease activity of the allergen since it was not observed in the absence of DTT (n=12), and was completely suppressed in the presence of 10 nM E-64, a specific inhibitor of cysteine proteases (n=12). D. pteronyssinus-induced increase in epithelial conductance could be entirely attributed to an increase in the paracellular conductance (+11.2+/-1.2%, n=8, p<0.01). There was no electrophysiological evidence of rupture in epithelial continuity, and no cell detachment was observed on microscopic examination. In conclusion, the cysteine protease activity of crude Dermatophagoides pteronyssinus extract is able to increase the epithelial paracellular conductance of rat tracheal tissues, even at relatively low doses that do not induce cell detachment or cell death.