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1. The contralateral bulbar pyramids were explored with low impedance micro-electrodes in cats anaesthetized with chloralose to reveal the effect of Group I afferent volleys (deep radial nerve of the forelimb) on pyramidal tract (Pt) cells.

2. Low rate (0·5/sec) stimulation of Group I afferents produced small responses (5-30 µV) in the bulbar pyramid which could be detected only with response averaging methods. The responses appeared with an initial latency of 7·0-11·2 msec and reached peak amplitude in 15·7 msec (mean latency). The pyramidal tract origin of the potential was demonstrated by its depression at stimulus rates above 1-2 sec and its disappearance at rates above 4/sec.

3. Recordings of neurones in the Group I cortical projection zone of the posterior sigmoid gyrus revealed that several types of cells, including Pt cells, were activated by Group I afferent volleys.

4. Pt cells responding to Group I afferent volleys frequently received convergent actions from low threshold cutaneous nerve volleys.

5. Averaged response recordings from electrodes positioned in the medial portions of the lateral funiculus of the spinal cord at the level of C₂, revealed a response to Group I afferent volleys as early as 7·4 msec which possessed the same characteristics as the relayed response to Group I in the bulbar pyramids. Some Pt cells, activated by Group I volleys orthodromically, could also be antidromically activated by stimulation of the recording site in C₂.

6. It was concluded that group I afferent volleys can influence, after short latencies, Pt and non-Pt cells and that some of these Pt cells gave rise to axons incorporated in the corticospinal tract.