The numbers, laminar position, perikaryal and dendritic morphology, and axonal trajectories of postsynaptic cells ascending the dorsal column have been studied after implantation of HRP pellets in either the dorsal columns or dorsal column nuclei after destruction of the dorsolateral fascicle on one side. Observations made throughout the spinal cord gave estimated figures of 800-1000 and 1700-2000 cells in lumbosarcal and brachial enlargements respectively on the side of the implant. The commonest type (C), centred on lamina IV, had dendritic trees greatly extended rostrocaudally and restricted mediolaterally in the lateral dorsal horn, the extension and restriction diminishing for more medial cells. Type B cells differed dramatically, with large straight dendrites in the transverse plane and large perikarya in medial lamina V. Type A cells, distinguished by both rostrocaudal and mediolateral restriction in dendritic trees, were only found medially in laminae III and IV. Outside the enlargements, in high lumbar and thoracic cord, many fewer cells were found, corresponding to Type C but with dendrites much elongated rostrocaudally and little mediolateral variation. Many small fusiform cells were found in medial lamina VI in the upper cervical cord, distinct from any of the above. A few cells were found in the cord enlargements in lamina VII of the contralateral ventral horn, with axons crossing through the ventral commissure. The axons of all cell types were tortuous, and some entered the dorsolateral fascicle before crossing into the dorsal column: collaterals were often seen but could not be followed far. A complementary study of cells with axons ascending in the dorsolateral fascicle is reported in the following paper.