Six lamina III interneurones of the cat spinal cord were impaled and stained with intracellular injections of horseradish peroxidase. The responses of these neurones varied considerably: 1 neurone responded to light brushing of its receptive field, whilst 4 cells were excited by strong pressure. Morphologically, they were also a heterogeneous population. Two neurones had rostro-caudally orientated dendritic arbors that were confined to the lamina, while four of the cells were vertically orientated and possessed dendrites that crossed lamina boundaries. There was no correlation between the physiological characteristics of a neurone and its morphology. Three of the vertically orientated neurones were examined ultrastructurally. The first of these cells received several types of synaptic terminal which were distributed in an organised pattern over the entire dendritic tree. This neurone possessed recurrent dendrites which participated in a variety of complex synaptic arrangements. The second neurone also participated in a variety of synaptic arrangements, including glomeruli in lamina II, and received contacts from vesicle-containing dendrites. It gave rise to collateral axons which arborized in lamina II and formed boutons which formed synapses with dendrites. The third cell possessed varicose dendrites which were probably artifactual. It is concluded that lamina III interneurones are a heterogeneous population by electrophysiological, morphological and ultrastructural criteria. They differ in many respects from lamina II neurones and from the cells of origin of ascending systems. The diversity of their inputs and their variation in morphology suggests that they receive input from a variety of primary afferent fibres and dorsal horn neurones and hence may integrate information from these sources.