The effect of serotonin on dog white matter ultrastructure in the CNS was investigated. The intracysternal injection of this amine (6 microgram in 0.1 ml of physiological saline) induced significant lesions in the myelin and glia of the spinal cord white matter areas adjacent to the cerebrospinal fluid tracts. A loss of the regular structure and stratification of the myelin lamellae, as well as rupture and lysis of the myelin membranes, and their demyelination occurred. There was vacuolar dystrophy of oligodendrocytes; the astrocytes remained practically unchanged. After local intracranial injection of this amine (2 microgram in 0.01 ml of physiological saline) similar lesions were observed in the white matter of the cerebral hemisphere; in addition signs of inflammatory reaction were revealed at the late period of investigation. In control animals injected with physiological saline the changes were observed later and in gliocytes only. A conclusion was drawn that serotonin was capable of injuring the myelin and glia.