Studies have been made on the fine structure of HRP-labeled primary afferent endings in segment IX of the developing spinal cord during formation of direct sensory-motor contacts. It was found that single dorsal root collaterals reach the lateral motor nucleus and form synapses on motoneuronal bodies and their dendrites. In some of the synapses, two types of junctional specialization, i.e. active zone and gap junction, were observed.