1. Field potentials and single unit discharges were recorded in the dorsal accessory olive of the cat following stimulation of nerves of the contralateral hind limb.2. The largest field potentials were evoked when nerves were stimulated with brief trains at 200-300/sec. Responses were sometimes evoked from quadriceps nerve by volleys restricted to low threshold group I afferents, but responses were not usually evoked from other muscle nerves unless both group I and II afferents were excited.3. The spatial distributions of the field potentials evoked from different nerves were very similar and individual units were made to discharge by stimulation of each of a wide range of nerves including muscle, skin and joint nerves. Single units were also made to discharge by stimulation of both the ipsilateral and the contralateral quadriceps nerve.4. The latency of the field potentials evoked by stimulation of quadriceps nerve ranged from 14 to 23 msec in different preparations. The latency of responses evoked from various branches of the sciatic nerve was consistently longer and was usually between 20 and 30 msec.5. The activity was conveyed by a tract which, at the upper lumbar level of the spinal cord, lay in the ventral quadrant ipsilateral to the olive. In the upper cervical region the tract lay close to the ventral surface of the cord. Values between 24 and 30 m/sec were found for the conduction velocity of the tract.6. An excitatory response in the dorsal accessory olive was followed by a period of about 100 msec during which the amplitude of a second response was reduced.