Methylene Blue dye spraying method is currently used in the endoscopic assessment of the colonic mucosa of patients with chronic ulcerative proctocolitis. We have assessed the validity of this method in the evaluation of the functional capacity of the rectal mucosa in 17 patients with ulcerative proctitis and in 5 normal subjects. The ionic transmucosal fluxes of Na+ and K+ and the potential difference were used as parameters of the mucosal function. Our results indicate that methylene blue dye spraying method can be useful in the functional assessment of the rectal epithelium, especially for differentiating normal subjects and patients with mild ulcerative proctitis.