The fine structure of neurones belonging to the Postsynaptic Oorsal Column (PSDC) system and the primary afferent fibre types known to project onto them in the cat was studied using intracellular and retrograde labelling with horseradish peroxidase. Both intracellularly and retrogradely labelled neurones were selected for ultrastructural analysis. Four types of bouton were found in contact with P3QC somata and bendrites. (1) Boutons containing spherical, agranular vesicles forming asymmetrical synapses. These could also be observed terminating on other unlabelled profiles in the surrounding neuropil. (2) Boutons containing oval or pleomorphic vesicles (3) Boutons containing round agranular vesicles which formed the central terminal of glomerular complexes in laminae IIIII of the dorsal horn. (4) Large electron lucent terminals containing spherical agranular vesicles and forming multiple asymmetrical contacts onto P30C dendrites. Other complex synapses in which PSDC dendrites participated included axoaxonic and triadic arrangements. The P 3D C system is therefore more complex at the ultrastructural level than another ascending system (the spinocervical tract) which originates in the dorsal horn, and the functional implications of this complexity are discussed. Terminals from Group la, slowly adapting Type I (3AI) and rapidly adapting pad (RA) afferent units were also studied. Boutons formed by all three afferent types contained spherical agranular vesicles, however the Group la boutons were larger and generally formed fewer synapses with postsynaptic structures than the cutaneous axons. SAI and RA axons formed asymmetrical synapses onto dendritic shafts and spines and participated in both simple axodendritic and more complex arrangements involving other unlabelled dendrites and presynaptic axonal profiles. They differed in the average number of postsynaptic targets but the differences were not considered sufficiently great to allow identification of unlabelled primary afferent profiles on ultrastructural grounds alone.