Embryological and electron microscopic studies were conducted on pyramidal cells in the C(A4) area of the hippocampus of rats. In rats 1-3 days after birth, a few mitochondria and poorly developed Golgi apparatus were noted in the cytoplasm of pyramidal cells. In rats 5 days after birth, a few mitochondria, moderately developed Golgi apparatus and a small amount of rER were noted in the cytoplasm of pyramidal cells. At 7 days after birth, mitochondria, Golgi apparatus and rER were slightly better developed than those 5 days after birth. From this stage on, it became possible to observe a structure similar to the apical dendritic process of pyramidal cells. On the ninth day after birth, mitochondria and Golgi apparatus were not very well-developed, but rER was considerably well-developed within the cytoplasm near the two poles of the nucleus. At this srage, apical dendritic processes, with synapses on the surface was noted. On the 12th day after birth, mitochondria were distributed throoghout the cells. Golgi apparatus were also well-developed. The number of apical dendritic processes had increased, and 2-3 large synapses were noted on the surface of these cell membranes. On the 14th day after birth, the distribution of mitochondria, degree of development of Golgi apparatus and distribution of rER became similar to those in pyramidal cells of mature rats.