The structure of the roof of the fourth cerebral ventricle has been studied by scanning and transmission electron microscopy in Rana pipiens from the late embryonic stages, through the larval stages to metamorphosis. Stage 20 and 21 post-hatching embryos have a rhombencephalic roof consisting of undifferentiated embryonic cells. From embryonic stage 22-25 there is progressive differentiation into choroid plexus and posterior tela ependymal cells. Between 1 and 3 interependymal pores occur in the posterior tela and there is a small incipient subarachnoid space dorsal to the posterior tela. During the larval stages, total pore number increases with larval length in a curvilinear manner, with the steepest increase occurring in the latter part of the larval stages. By metamorphosis, total pore number (50-100) approaches that found previously in the adult amphibian. The diameter of individual pores ranges from 1 to 200 microns. Small pores are more numerous than large pores at all stages and both small and large pores become more numerous as development proceeds. Large pores may form by the fusion of two or more smaller pores. It is concluded that pore formation is a gradual process with its onset in the late embryonic stage and concurrent with the onset of choroid plexus and subarachnoid space development. It is suggested that the onset of CSF secretion and circulation may take place shortly after hatching.