1. In anesthetized monkeys, stimulation of muscle afferents results in a sequence of cord dorsum potentials. These include a group I volley followed by several negative potentials called here the NI, NII and NIII waves. 2. Evidence based on the effects of graded stimulus strengths, measurements of latencies, and the results of anodal blockade of large muscle afferents indicate that the NI, NII and NIII waves are evoked, respectively, by group I, II and III muscle afferents. 3. The NII and NIII waves appear to be confined to the lumbosacral enlargement when evoked by hind limb muscle afferents. However, the group I volley and NI wave can be detected at least as far rostrally as L 3. 4. The NII and NIII waves were mapped in the depth of the cord. The maxima for these waves were found in the neck of the dorsal horn. The waves reversed to become positive in the ventral horn. 5. Using graded electrical stimulation of muscle nerves it was possible to demonstrate that a few spinothalamic tract neurones could be activated monosynaptically by group I volleys; other spinothalamic cells may have been activated polysynaptically by group I volleys. The lack of any substantial excitation of spinothalamic neurones by intra-arterial injections of succinylcholine suggests that these group I actions may have been due to group Ib afferents from Golgi tendon organs. 6. The most potent excitation of spinothalamic tract cells was due to the action of middle sized and small muscle afferents. Evidence was obtained for an excitatory action of group II, III and IV afferents. There was a good correlation between the effects of graded stimulation in evoking discharges in separate bursts associated with the arrival of volleys in group II and group III afferents and the generation of the NII and NIII waves. 7. Some spinothalamic neurones, including several located in lamina I, were unaffected by the muscle afferent volleys used. It is suggested that such neurones might help to signal well localized pain, whereas the cells which respond to a variety of cutaneous and muscle afferents might be involved in signalling poorly localized pain which is subject to referral.