The fine structure and synaptic features of the ruffed cell, especially those of the initial unmyelinated portion of its axon (IP), were investigated by means of electron microscopy. The round or oval cell body is 10 to 20 micrometers in diameter. Near the cell body, there is a specialized region of the IP, about 20 to 40 micromerters in diameter, which consists of branched protrusions from the IP. Many neuronal processes end on these protrusions, and the whole assembly reminds one of a bird's nest. The nest has a little higher electron density than the surrounding field. The nucleus of the cell itself is round or oval with irregular undulations, and measures 7 to 10 micrometers in diameter. The perikaryon contains the usual cell organelles; especially many clusters of free ribosomes. Two kinds of dendrites arise from the cell body: thick dendrites, about 4 micrometers thick, which appear to be extensions of the perikaryon; and thinner dendrites, about 1 to 2 micrometers in diameter. In addition to these two, processes resembling those of glia arise from the cell body or from the thick dendritic trunk. Some of the dendritic branches enter the glomerulus. The IP arises from the cell body as a protrusion-bearing process about 0.8 to 1.5 micrometer in diameter. The IP is divided into three parts: part 1, where many elaborate protrusions arise to constitute a nest; part 2, where several scattered protrusions arise; and part 3, which has no protrusions. Part 1 of the IP is further subdivided into three portions according to the fine structure of its shaft. The first portion, about 10 to 15 micrometers long, is rather straight and exhibits the two characteristics of the conventional initial segment, i.e., membrane undercoating and fascicles of microtubules. The second portion, about 10 to 20 micrometer long, shows the membrane undercoating, but no fascicles of microtubules. The third portion of part 1 as well as parts 2 and 3 exhibits neither of the two distinguishing characteristics of the initial segment. Synapses are encountered on the cell body, the dendrites, and the IP. Most of them are formed with granule cell dendrites. The ruffed cell is presynaptic in asymmetrical synapses whose postsynaptic elements are the granule cell dendrites and other kinds of neuronal processes of unknown origins. It is also postsynaptic in symmetrical synapses from the granule cell dendrites. Reciprocal pairs of these two types of synapses are also seen, both on the dendrite and the IP. The numbers of synapses on the dendrite and the cell body seem far less than on the IP. The number of synapses on one IP is roughly estimated to be 1,000 to 2,000. The ratios of the synaptic types are as follows: asymmetrical synapses from the IP, 63%; symmetrical synapses onto the IP, 12%; and reciprocal pairs of synapses, 25%. Gap junctions are also seen between protrusions of the ruffed cell IP and dendrites of the perinest cell, which is a small neuron located at the periphery of the nest...