AbstractPresynaptic depolarization of group I afferents produced by volleys in contralateral hind limb nerves has been investigated. It was found that a train of volleys in contralateral muscle or skin nerves did not alter the excitability of the Ia fibres or the size of the monosynaptic EPSP in motoneurones, i. e., there is no contralateral presynaptic inhibitory action onto group Ia fibres. A train of volleys in contralateral muscle and skin nerves increased the excitability of the contralateral Ib fibres to the flexors and extensors. The time course of the increased excitability had a latency of 10–30 msec, a slow rising phase, a late maximum at 50–80 msec, and a duration of about 150 msec. The crossed presynaptic actions originated from high threshold (group II and III) muscle afferent and cutaneous fibres. The results also indicate that Ib fibres may have a depolarizing action onto the contralateral Ib nerve termin