The sensomotor cortex and nucleus caudatus of the embryonal (14-22 days) and newborn rats have been investigated by electron microscopic method. There were described two groups of microvesicles (10-20 nm in diameter) with smooth and rough external surface. They differ from other vesicular components of the developing synapses (such as synaptic vesicles, vesicles of growing conus and other) by the minimal size, structure and localization. Microvesicles with the rough external surface are found only at that moment when the membranous specialization of the further synapse from desmosomes begins to form. They are found along the membranes of active contact zone and are correlated with the degree of differentiation of these membranes. Microvesicles with the smooth external surface are found during the all period of synaptogenesis. They are found either in presynaptic terminal freely or they are attached to presynaptic membrane where they form cluster of microvesicles and they can be found on the surface of synaptical vesicles too. In this article the functional role and genesis of microvesicles are discussed.