Ultrastructural studies were carried out on the choroid plexus of eight species including amphibian, reptilian, and mammalian forms with special attention directed toward the phenomenon of apical blebbing in choroid plexus epithelial cells. Choroidal blebs in various stages of formation, release, and maturation were observed in all species examined. Evidence presented by this and previous studies suggests that choroidal and ependymal blebbing represents a physiologically significant mechanism by which proteins and/or other molecules enter the cerebrospinal fluid (CSF).