In order to analyse whether or not surface morphology and intracellular skeleton are interdependent, Cytochalasin B (CB), an agent which interferes with microfilaments, was introduced ino the brain ventricular system of adult rats. Following intraventricular application of CB for different periods of time (up to 6h) the animals were sacrificed and the ventricular wall examined with both transmission and scanning electron microscopes. The first signs of cellular alteration after short periods of CB application were an increase in the number of microfilaments within the cytoplasm and the appearance of a multitude of small knobs on the surface of the cilia of the cuboidal ependymal cells, whereas the tanycytes exhibited extensive blebbing at their apical poles. When the duration of CB perfusion was lengthened, entire ciliated cells became rounded up and were loosened from the epithelium, especially in the transition zone of the third ventricle where the ependyma is composed of both tanycytes and ciliated cells. The significance of the CB-induced changes in the ependyma are discussed in relation to the ependymal surface modifications seen under physiological and other experimental conditions.