This work was undertaken to study the myelination and cholinesterase activity of the developing white matter of chick embryo spinal cord from the 6th day of incubation until hatching. Myelination of the white matter was always preceeded by the aggregation of neuroglial cells. The sequence of myelination in the cervical and thoracic segments was from before backwards, whereas in the lumbosacral segments it was from behind forewards. Myelination of the ventral column extended craniocaudally, whereas in the posterior column is extended caudocranially. True cholinesterase activity was encountered in the spinal white matter only before and during the process of myelination and was closely associated with the sites of neuroglial aggregations. No pseudocholinesterase activity could be seen in the white matter before myelination. However, during the period of active myelination, enzymatic activity was seen only at the sites of formed myelin and after myelination of all tracts; the whole white matter reacted positive for the enzyme. The significance of these findings is discussed.