The dye Evans blue was used to monitor vascular protein leakage. Fluorometric measurement of Evans blue in formamide extracts of rat tracheal tissue was performed after induction of protein leakage by electrical vagus nerve stimulation and compared with the widely used colorimetric detection. The fluorescence method was approximately 100 X more sensitive than the colorimetric method. Furthermore, Evans blue fluorescence (excitation at 620 nm, emission at 680 nm) was used for microscopic investigation of cryostat sections of tracheal tissue. Extravasated Evans blue after electrical nerve stimulation was mainly found in the subepithelial layer of the trachea obviously bound to tissue constituents. It is suggested that Evans blue fluorescence can be applied for quantification of protein leakage with high sensitivity which opens the possibility of measuring this reaction in very small regions with high accuracy, as well as for tissue localization of protein leakage at the microscopic level.