The permeability of the ovarian follicle in Locusta migratoria has been studied by incubating the gonad in vitro with horseradish peroxidase (HRP) as an electron-dense tracer. The exogenous protein freely penetrates as far as the perioocytic space by interfollicular channels during all stages of oogenesis. Follicular cells are only linked by desmosomes and gap junctions. During phase II of vitellogenesis, intercellular spaces are conspicuous and almost all junctions disappear. After the vitelline envelope is deposited, the lateral membranes again become apposed and the previous junctions are reconstituted. Simultaneously, septate junctions appear, and by the end of chorion deposition they have grown from the apex up to half the height of the follicular epithelium. Permeability to peroxidase is blocked by septate junction formation. Tight junctions were never found at any stage of follicle development. Observation of freeze-fractured replicas supports the hypothesis that septate junctions constitute the only morphological evidence for the existence of a permeability barrier.