That synaptic vesicles with a diameter of about 50 nm could be demonstrated by the freeze-fracturing technique was an important contribution to the knowledge of synaptic fine structure (Moor, Pfenninger & Akert 1969; Akert, Moor, Pfenninger & Sandri 1969). The finding u spheric profiles (figures 1, 3 a, c) in rapidly frozen tissue renders an artefactual origin of vesicles very unlikely. However, the chief advantage of freeze-fracturing lies in the visualization of membrane surfaces which are of special interest in presynaptic nerve terminals. The present micrographs were obtained from freeze-fractured monkey, cat, and rat spinal cord, cerebral cortex, and subfornical organ as well as from pigeon optic tectum. Special attention was paid to areas in which large pieces of membranes remained attached to the surfaces of nerve cells (figures 1, 3 a ). This situation suggests a special type of affinity between the two elements, and the association of one of the membranes with clusters of synaptic vesicles indicates that presynaptic membranes may be involved.