The role of tight junctions in permeability barriers is now well established. Other roles for this junction have been postulated but have not been demonstrated. In both adult and prepubertal dogs, stimulation with human chorionic gonadotropin (hCG) results in an increased number of pinocytotic vesicles on the membrane faces enclosed by tight junctions within the inter-Sertoli cell junctions of the testis. It is suggested that these membranous regions, protected from the normal flow of intercellular fluids, may act as regions of selective intercellular exchange.