Portions of the superior sagittal sinus and lacunae laterales containing arachnoid villi and granulations from 8 full-term newborn babies were studied by transmission electron microscopy. Arachnoid proliferations from 3 subjects were distended and fixed in vitro by applying a differential pressure of 8 cm H2O to the subarachnoid aspect of the tissues. The remaining cases were fixed in a collapsed state. Distended arachnoid proliferations showed morphologic characteristics associated with similar functional structures in experimental animals: shortened and enlarged interendothelial spaces; micropinocytotic activity and a system of endothelial-lined tubules. All this is taken as evidence that arachnoid proliferations in newborn babies could already be engaged in cerebrospinal fluid absorption.