Abstract AIM: Members of the tight junction protein family of claudins have been demonstrated to specifically determine paracellular permeability of the intestinal epithelium. In small intestinal mucosa, which is generally considered to be a leaky epithelium, Peyer's patches are a primary part of the immune system. Aim of the study was to analyze the tight junctional barrier of follicle-associated epithelium covering Peyer's patches lymphoid follicles. METHODS: Employing small intestinal tissue specimens of male Wistar rats, electrophysiological analyses including the Ussing chamber technique, marker flux measurements and one-path impedance spectroscopy were performed. Morphometry of HE stained tissue sections was taken into account. Claudin expression and localization was analyzed by immunoblotting and confocal laser scanning immunofluorescence microscopy. RESULTS: Almost two-fold higher parameters of epithelial and transepithelial tissue resistance, and a markedly lower permeability for the paracellular per