It has been suggested that synaptic vesicles, seen in great profusion in electron micrographs of many different types of nerve ending as small circularprofiles about 005p in diameter (Sj6strand, 1953; De Robertis, 1958), are storage particles for transmitter substances, among them acetylcholine, and provide a morphological basis for the quantization of transmitter release detected electrophysiologically at the motor end-plate (del Castillo& Katz, 1956; Palay, 1956). The isolation of these vesicles as a distinct fractioncontainingtransmitter substance would be valuable evidence for this theory. It would in addition provide a useful invitro preparation for the study of transmitter release and the action ofdrugs and toxins on this process. Hebb & Whittaker (1958) and Whittaker (1959) have succeeded in isolating from sucrose homogenates of forebrains of rabbit, guinea-pig and other species a particulate fraction (fractionB, Whittaker, 1959) distinct from nuclei, mitochondriaand microsomes, and containing most of the bound acetylcholine (Hebb & Whittaker, 1958; Whittaker, 1959), hydroxytryptamine (Whittaker, 1959) and noradranaline (Chrus' ciel, 1960) of the tissue. Preliminary electron-microscopic studies by KM Smith and GJ Hills (reported by Whittaker, 1959) suggested that the particles of this fraction might be derived from synaptic vesicles. Fraction B is one of three obtained by the density gradient separation of a crude mitochondrial (P2) fraction of brain. The other two are an unidentified, lighter, inactive fraction (A) and a denser fraction (C) identified on biochemical and mor-phological evidence as mitochondria.

The primary object of the present work was to investigate the morphology of the particles of fraction B in greater detail using improved techniques offixing, staining and embedding which have proved particularly useful in electron microscopic work on the central nervous system (Gray, 1959). The conditions necessary for obtaining reproducible electron micrographs were worked out and a general study was made of all the fractions obtained by the homogenization and centrifugation procedure. The results (briefly reported at the IV International Neurochemical Symposium, Varenna, 12-17 June 1960 (Whittaker, 1961) and by Gray & Whittaker, 1960) show