Abstract The study was aimed at the investigation of the rat corticospinal system both functionally and anatomically using as a functional marker the immediate early gene c- fos, combined with retrograde tracing with horseradish peroxidase (HRP). This was achieved by mapping c- fos induction immunocytochemically in the spinal cord as a result of occlusion of the middle cerebral artery (MCA). Following left-sided MCA occlusion, Fos-like immunoreactivity (Fos-LI) was localized in both the dorsal and ventral horn neurons at the lumbar segment of the spinal cord. Labelling was often bilateral but was generally more substantial ipsilaterally. In the ventral horn, some of the Fos-positive neurons were confirmed to be somatic motoneurons innervating the tibialis anterior muscle of the lower extremity contralateral to MCA occlusion, as shown by their retrograde labelling with horseradish peroxidase injected into the muscle. Fos-LI was absent in the ventral horn of the spinal cord at cervical, thoracic, and sacral segments in both experimental and sham-operated rats. These findings suggest that the expression of c- fos may be used as a sensitive transneuronal marker for the study of neuronal activity in the spinal cord elicited by brain damage, viz. focal cerebral ischaemia, and when coupled with injection of HRP as a retrograde tracer, the method may prove to be useful for the study of transneuronal effect of the damage of the corticospinal motor system. While the expression of c- fos in the spinal motoneurons was most probably attributable to transneuronal effect following MCA occlusion, the possibility of that c- fos can be induced by altered hindlimb activity after the cerebral ischaemic insult cannot be excluded.