Serotonin is a normal constituent of the brain and has been claimed to act as a neurohormonal or regulatory agent in nerve transmission. Previous workers have only studied its whole brain or subcellular particulate distribution. Because the brain itself is divided into many distinct ganglia and nuclei, the distribution of serotonin in these sites is considered to be of greater importance than either the whole brain or subcellular distribution. This paper is a study of the distribution of serotonin in particular brain sites in rats with normal and elevated levels of the amine, with and without electroshock treatment. The results indicate: (1) a heterogenous distribution of serotonin and 5-hydroxytryptophan in the rat brain; (2) serotonin and 5-hydroxytryptophan are transported down the neuron axon; (3) serotonin and 5-hydroxytryptophan are not utilized by all neurons; (4) iproniazid and/or electroshock can alter the uptake, distribution and transportation of serotonin and 5-hydroxytryptophan in the different areas and fibers of the central nervous system.