AbstractThe crude mitochondrial fraction P2and subfractions of P2were prepared from the brain stem, hemispheres and whole brain of 19‐day‐old fetal rats. Samples were fixed in glutaraldehyde‐osmium, NaMnO4or by Tranzer's triple fixation method (aldehyde‐chromate‐dichromate‐osmium) and examined by electron microscopy. The C‐fraction from whole brain was the main synaptosome fraction, containing 3.2 % presynaptic terminals as counted from all membrane bound particles. The brain stem showed more presynaptic terminals than the hemisphere (2.8% versus 0.9%) suggesting a caudal‐rostral maturation gradient for synaptogenesis. The maturity of the nerve endings obtained was very variable in contrast to the rather uniform synaptosomes derived from adult tissue. They varied from profiles without any substructures to mature synaptosomes displaying asymmetric synaptic junctions. Monoamine synaptosomes containing small granular vesicles were not detected in the present study, suggesting immaturity of the granular monoamine pool at this stage