Responses of spinal cord neurones to excitatory input from myelinated and non-myelinated afferent fibres of the knee joint were investigated in spinalized cats that were anaesthetized with alpha-chloralose. The hind limb was largely denervated except for the knee joint. The action of joint afferent volleys on populations of spinal cord neurones was assessed by recordings of cord dorsum potentials and of field potentials within the substance of the spinal cord. Three different negative cord dorsum potentials (NI, NII and NIII waves) were produced by volleys in progressively smaller sized myelinated afferent fibres of the posterior articular nerve of the knee. Volleys in non-myelinated joint afferent fibres did not evoke a detectable cord dorsum potential. The NII and NIII waves could be recorded at segmental levels L4-S2. Field potentials corresponding to the cord dorsum negative waves were recorded with a tungsten micro-electrode inserted into the substance of the cord. The potentials were negative when the recording electrode was in the dorsal horn, but reversed to become positive in the ventral horn. There were differences in the depths at which the different components of the field potential sequence reversed in sign. Single unit recordings revealed that afferent volleys in the posterior articular nerve could excite neurones in segments L5-S1 and at depths from less than 1 to 4 mm below the dorsal surface of the cord. Most marked recording sites were in laminae I, IV-VI or VIII. Many units were activated just by the A fibre component of the joint afferent volley, but others could also be excited by the C fibre component. None were excited just by C fibres. All units tested had a convergent excitatory input from fibres belonging to cutaneous, muscle and mixed nerves. Neurones excited by joint afferent volleys were tested for a receptive field using several forms of local mechanical stimulation of the knee joint and joint movements. Most cells could be activated by one or more of these stimuli. Although many of the effective mechanical stimuli were innocuous, noxious stimuli could often excite the cells more effectively, and some neurones were only activated by noxious stimuli. A few cells with receptive fields in the knee joint did not respond to joint movements.