IT is well known from the work of Lloyd and others1 that stimulation of muscle nerves with shocks that activate fibres smaller than group I generally facilitates ipsilateral flexor reflexes and inhibits ipsilateral extensor reflexes. However, several early investigations demonstrated that the ipsilateral input to extensor muscles, from both skin and muscle, must contain an excitatory component2. Recent investigations have provided further evidence for the presence of such excitatory connexions. Thus Hagbarth3 showed that stimulation of appropriate skin areas would facilitate ipsilateral monosynaptic extensor reflexes; Wilson, Diecke and Talbot4 showed that tetanus toxin causes polysynaptic reflexes to appear in various extensor nerves in response to dorsal root stimulation; and Holmqvist and Lundberg5 occasionally observed facilitation of the monosynaptic reflex in the nerve to gastrocnemius by stimulation of high-threshold afferents from several muscles. Finally, some evidence for ipsilateral excitatory effects in extensors is available from intracellular investigations. Eccles and Lundberg6 have observed, in ‘Nembutal’-treated low spinal cats, excitatory post-synaptic potentials in extensor motoneurones following stimulation of high threshold fibres in ipsilateral muscle nerves; such findings were rare. The present experiments demonstrate, by means of intracellular recording, that in decapitate unanaesthetized cats ipsilateral excitatory actions are found frequently in certain extensor motoneurones.