The choroid plexus and its associated epiplexus cells in the fourth ventricle in cats were studied with scanning and transmission electron microscopy (SEMTEM) following a cisternal injection of crotoxin complex (phos-pholipase A2). In SEM the epiplexus cells of the control animals were predominantly stellate with long radiating processes. At 2 h after the administration of crotoxin complex, these radiating processes flattened out forming sheet-like membranes covering the ventricular surface of the choroid epithelial cells. The membranous coverings remained extended in 5-hour-survival cats. Numerous blebs of different sizes were observed in areas that were not covered by the cytoplasmic membrane in 5-hour animals. Some of the blebs appeared to have ruptured. InTEM, the microvilli of the choroid epithelial cells in crotoxin complex-treated rats were dilated. The luminal surface of the epithelial cells showed eruption of blebs filled with amorphous materials. Pinocytotic vesicles increased in number in the apical cytoplasm. The lumen of the ventricle often contained portions of cytoplasm believed to be derived from the extrusion of the blebs.These appeared to be engulfed by the overlying epiplexus cells. It was concluded that the injected crotoxin complex stimulated both the secretory as well as pinocytotic activity of the choroid epithelial cells. The phagocytosis of the secretory products from the epithelial cells by epiplexus cells suggests a close functional relationship between the two cell types.