1. To see how muscle group III and IV receptors are affected by a myositis, the background activity and mechanical excitability of slowly conducting afferent units from normal and inflamed muscles were studied in chloralose-anesthetized cats. The inflammation was induced by infiltrating the gastrocnemius-soleus muscle with a suspension of 2% carrageenan. According to their responsiveness to local pressure stimulation the receptors were classified as touch units, moderate pressure units, and noxious pressure (probably nociceptive) units. The impulse activity in single afferent units was recorded up to 14 h after induction of the inflammation. 2. In inflamed muscle both group III and group IV receptors showed an increase in the proportion of units having a background activity and in the mean background activity. The differences reached statistically significant levels in group III fibers only. 3. A characteristic feature of the background activity of some receptors in inflamed muscle was its intermittent nature: the discharges occurred either as grouped impulses of short duration or as phases of relatively high discharge frequency alternating with long periods of silence. 4. In normal muscle no receptor exhibited intermittent discharges or had a discharge rate exceeding 7 imp/min. Thus the presence of an intermittent background activity or a high frequency of the background discharge can be considered as characteristic for afferent units from an inflamed muscle. 5. The time course of the background activity showed two peaks, one occurring 2-4 h, the other one 6-7 h after induction of the inflammation. Recordings of single units during the transition from the normal to the inflamed state demonstrated that the first increase in background discharge took place 1-1.5 h after injection of carrageenan. 6. The proportion of noxious pressure units was reduced and that of moderate pressure units increased in inflamed muscle. In this case the difference was significant for group IV units only. 7. The response curves upon mechanical stimulation did not show significant differences in normal and inflamed muscle, i.e., sensitized noxious pressure units behaved similar to real touch units or moderate pressure units with respect to their mechanical excitability. 8. No apparent correlation existed between the receptors' mechanical threshold and degree of background activity in inflamed muscle. This suggests that the inflammation-induced change in mechanical excitability and development of background activity are independent phenomena.(ABSTRACT TRUNCATED AT 400 WORDS)