In the central nucleus of the cat's brain inferior colliculus, three classes of neurons were observed. 1. Large relay neurons were distinguished by the abundance of cytoplasmic organelles. A considerable part of their perikaryon was covered by presynaptic terminals. 2. Medium sized lucent neurons having small amount of organelles. 3. Dark cells of rare occurrence in the central nucleus, which according to the number of axo-somatic synapses may be placed between type I and II neurons. In the central nucleus four type of profiles containing synaptic vesicles were distinguished. Large polymorphic endings of specific afferent fibers (type I) and small neurons (type II) contained round synaptic vesicles, while type III irregular axon terminals contained flattened and polymorphic ones. Type I, II and III terminals always appeared to be presynaptic components of synapses. Type IV terminals were found to be rare and except the polymorphic synaptic vesicles contained profiles of endoplasmic reticulum, ribosomes, microtubules, that enables them to be attributed to dendrites. Type IV terminals may be either presynaptic, or postsynaptic components of synapses.