Each seventh cervical dorsal nerve root is attached to the spinal cord surface by four to eight rootlets. A tapering outgrowth of central nervous tissue, the central tissue projection, extends distally into the proximal part of each rootlet in the immediate postnatal period. The central ends of the most proximal peripheral internodes surround this projection. Thus a length of rootlet contains both CNS and PNS tissue. This is termed the transitional zone. Material was processed by standard preparative techniques for electron microscopy. Serial semithin and ultrathin sections were made over the entire extent of several transitional zones at ages ranging from 2 to 300 days postnatum. Central tissue projections were reconstructed in three dimensions and analysed morphometrically. The morphology of the central tissue projection varies during development. At first, it forms an irregular projection into the anterior portion of the rootlet. It than elongates and takes the form of a dorsoventrally flattened, distally tapering wedge. By 20 days postnatum it has attained its definitive form. This consists of three segments: a proximal wedge-shaped portion, similar to that described above; continuous with this is a distally tapering, dorsoventrally flattened, cone-shaped segment which generally branches into two or more slender projections of central tissue. The latter comprise the third segment. The projection comes to form a substantial proportion of the anterior, proximal and distal surfaces of the dorsal rootlet from an early stage. The mean length of the central tissue projection increases progressively over all intervals studied, except that between 12 and 30 days postnatum, when a reduction in length is associated with reorganisation of the morphology of the projection. Projection length varies considerably between rootlets and is relatively weakly correlated with rootlet cross sectional area. There is a great deal of overlap between the distributions of projection lengths at all stages between 20 and 300 days.