Low-threshold afferents from human limb muscles are known to project to the sensorimotor cortex and to contribute to proprioception. However, there are few data on the cortical projection of afferents from human respiratory muscles. The present study employed evoked-potential techniques to determine whether low-threshold muscle afferents from the chest wall project to cortical levels in conscious human subjects. In four subjects intramuscular afferents of the second parasternal and fifth lateral intercostal muscles were selectively stimulated through an insulated microelectrode inserted percutaneously at the respective motor point. Evoked potentials were recorded and averaged from eight scalp sites. The initial cortical component of the cerebral response to intramuscular stimulation of the second and fifth interspaces was a negative potential commencing at 19.2 +/- 2.1 msec and 20.7 +/- 1.1 msec respectively. The dominant early cortical potential was largest at the vertex, and was comparable in amplitude (0.58 +/- 0.23 microV) to that for individual muscles of the upper and lower limbs. The cortical focus was distributed differently from that for cutaneous afferents of the chest wall and for both muscle and cutaneous afferents from the upper and lower limbs. This study provides direct evidence for a short-latency projection from intercostal muscle afferents (group I and/or II) to the human cerebral cortex.