In the spinal cord of cats, simultaneous recordings were made from single Renshaw cells (extracellular) and single motoneurones (intracellular). In a total of 5 cases, the two cells of such a pair showed mutual synaptic interactions: single motoneurone spikes excited the Renshaw cell, and Renshaw cell spikes caused the appearance of short-latency hyperpolarizing potential changes in the motoneurone. Unidirectional interactions were also seen: in two cases motoneurones excited Renshaw cells from which they received no inhibition, and in 7 cases Renshaw cells caused hyperpolarizing potential changes in motoneurones from which they received no excitation. The inhibitory effect of a single Renshaw cell onto a motoneurone was very weak (average 12.7 microV; range 1.5-54 microV hyperpolarization; n = 12.