The enhancing effects of bile salts on buccal penetration was investigated in vitro using porcine buccal mucosa, correlating permeability changes with histological effects. The permeability of the buccal mucosa to the model compound fluorescein isothiocyanate (FITC) was studied in the presence and absence of bile salts. Light microscopy, freeze-fracture electron microscopy and confocal laser scanning microscopy were used in order to investigate the interaction between the bile salts and the buccal epithelium. A significant increase in permeation of FITC was obtained after co-administration with bile salts. After 4h treatment, bile salts (at a concentration of 0.1M) caused a loss of distal layers in the epithelium and a split of the epithelium from the connective tissue. The results of freeze-fracture studies show that the bile salts affect the cytoplasmic domain of the buccal epithelium. Due to the bile salt treatment, the mode of fracture was altered in such a way that cell membranes were almost absent. However, no differences were observed between the enhancing effects of dihydroxy and trihydroxy bile salts, either with the transport rate or with the histological studies.