The effect of stimulation of the sensorimotor cortex on spinal reflex arcs was investigated in cats with intracellular recording from motoneurones and with conditioning of monosynaptic reflexes. Stimulation of cortex enhances the following synaptic actions in motoneurones: Ia inhibitory, Ib reciprocal, FRA reciprocal, FRA inhibitory to flexor and cutaneous excitatory to extensor motoneurones. The effects are mediated by the pyramidal tract which presumably exerts an excitatory action on interneurones of reflex arcs. Stronger stimulation of cortex evokes synaptic actions in motoneurones with excitatory action dominating in flexor and inhibitory in extensor motoneurones. There was frequently evidence of mixed excitatory and inhibitory effects in extensor motoneurones and inhibitory action in flexor motoneurones was also found. It is suggested that these actions on motoneurones are due mainly to activation of interneurones of spinal reflex arcs.