Using techniques for enhanced microtubular preservation (albumin pretreatment of Gray) occipital and pyriform cortices of rats were studied by electron microscopy at various stages of development. A close structural relationship was seen between microtubules (mts), focal membrane densities, synaptic vesicles, and presynaptic dense projections (pre-dps) during maturation. Mts were seen in the neonates to be focused onto the inner surface of immature axonic profiles merging or sometimes fragmenting there. Focal densities occur at these attachment sites and thereafter synaptic vesicles clothe the mts and abut onto these presumed primordial pre-dps. Thus, mts may contribute to the initial formation of pre-dps as well as their maintenance in the adult and may act to channel the first synaptic vesicles to the site of eventual synaptic contact and transmitter release.