Abstract Ependymal cells represent a major portion of the neuroglial population of the brain in lower vertebrates. Earlier investigations have suggested that thyroxin-dependent neuronal growth and maturation in the brain of metamorphosing amphibians are accompanied by proliferation and metabolic activation of the neuroglia. Electron microscopic studies of brains of Rana pipiens larvae reveal that, in the absence of the thyroid gland, ependymal cells lining ventricles of the developing amphibian brain display sparsely distributed ribosomes, few polyribosomes, and relatively simple profiles of rough-surfaced endoplasmic reticulum. When thyroxin is administered to such animals, free ribosomes and polyribosomes become numerous and stacks of lamellar ribosome-studded endoplasmic reticulum predominate. The observed changes are interpreted as reflecting an increased neuroglial capacity to synthesize export protein.