In 60 anaesthetized cats the discharge characteristics of single group IV afferent fibres from skeletal muscle at rest, during local mechanical stimulation of the muscle, and in response to injections of bradykinin have been studied. 250 group IV fibres from the gastrocnemius-soleus muscle have been identified by their conduction velocity. Of these fibres 121 have been recorded from the dorsal root and 129 from the muscle nerve. A resting discharge was present in 52% of the dorsal root fibres and in 57% of the fibres recorded peripherally. Out of 203 units subjected to local mechanical stimulation of the muscle 9 responded to light, innocuous pressure; 129 fibres required strong, presumably noxious stimulation to be excited. The rest of 65 fibres could not be activated by the mechanical stimuli used. Intraarterial injection of bradykinin excited 88 out of 184 group IV fibres. Intramuscular injection of bradykinin was also found to be an effective stimulus. External application of bradykinin to the intact and to the dissected muscle nerve regularly had no excitatory effect on fibres being sensitive to intraarterially administered bradykinin. Injections of vasoactive drugs (nylidrine, noradrenaline) failed to affect the activity of group IV fibres responding to bradykinin. It is concluded that bradykinin is a potent stimulant for the intramuscular endings of afferent group IV fibres; the effective concentration lying within the range observed in pathologically altered tissues. This sensitivity to bradykinin together with a high mechanical threshold suggests that muscular group IV afferent units participate in the reception of muscular pain.