Single group Ia muscle afferent fibres in the lumbar spinal cord of the cat were impaled with microelectrodes and labelled with horseradish peroxidase. Two collateral axons were prepared for combined light and electron microscopy. Arbors selected from lamina VI were processed by the postembedding immunogold technique with antiserum which specifically recognizes GABA in glutaraldehyde-fixed tissue. Twelve Ia boutons were examined through series of thin sections with the electron microscope and all of them were associated with presynaptic axon terminals which were positively labelled for GABA. Some Ia boutons received synaptic contacts from several GABAergic terminals. The present study establishes that a GABA-like substance is present in axon terminals presynaptic to Ia afferent boutons in lamina VI of the spinal cord. This evidence provides a morphological basis for presynaptic inhibition of Ia afferent input into lamina VI.