Paired intracellular recordings of interneurones and motoneurones have been performed in the lamprey spinal cord in vitro. One new type of small interneurone (10-15 microns in diameter) which produces monosynaptic inhibitory postsynaptic potentials (IPSPs) in motoneurones is described. The IPSP is strychnine-sensitive and thus presumably glycinergic. This type of interneurone undergoes rhythmical membrane potential oscillations during fictive locomotion and intracellular stimulation can have a profound effect on the burst generation occurring during fictive locomotion.