The morphology of single C3-C4 propriospinal neurones (PNs) including the cell body, dendritic tree, axonal trajectory and the pattern of projection and termination of axonal collaterals in the C3-C4 segments was investigated by intra-somatic or intra-axonal injection of horseradish peroxidase. All the C3-C4 PNs could be antidromically activated from the lateral funicle in C6 and the lateral reticular nucleus but not from Th13. Another criterion was that they received monosynaptic excitation from corticospinal fibres in the contralateral pyramid. Twenty-four C3-C4 PNs were successfully stained. They were located in the lateral part of laminae VI-VIII except for two neurones which were located in lamina V and two in lamina IX. Five to eleven dendrites originated from the cell bodies and extended throughout laminae IV-VIII and even into the white matter in the transverse plane and up to 3 mm rostro-caudally. The axonal trajectory from the cell body was usually curved before reaching the lateral funicle. The bifurcation of the stem axon into a descending and an ascending branch was mostly observed in the white matter close to or at the border between the white and grey matter at the level of the cell body. The ascending and descending axonal branches maintained their location in the same part of the lateral funicle. Sixteen out of 24 stem axons gave off collaterals in the grey matter and/or in the white matter. One to five collaterals were given off from the axons in the grey matter.(ABSTRACT TRUNCATED AT 250 WORDS)