Experiments were performed in barbiturate-anaesthetized, spinalized cats to investigate the effect of calcitonin gene-related peptide (CGRP) on the spatial distribution of immunoreactive substance P (ir-SP) in the spinal cord released by electrical nerve stimulation and noxious mechanical stimuli. The presence of ir-SP was assessed with microprobes bearing C-terminus-directed antibodies to SP. CGRP was microinjected into the grey matter of the spinal cord near microprobe insertion sites at depths of 2500, 2000, 1500 and 1000 μm using minute amounts (in total 0.2–0.5 μl) of Ringer solution containing CGRP at a concentration of 10−5 or 10−3 M. In the untreated cord electrical stimulation of the tibial nerve (suprathreshold for all C fibres) elicited release of ir-SP which was centred in and around the lamina II. After microinjection of CGRP, stimulation-associated ir-SP was detected in a region extending from the cord surface down to the ventral horn. This pattern was similar to that observed after the microinjection of synthetic peptidase inhibitors (Duggan et al., Brain Res., 579, 261–269, 1992). The large expansion of sites accessed by ir-SP was time-dependent, reaching a maximal effect within 10–40 min after microinjection of CGRP, and reversal was observed in subsequent probes. A similar expansion of the regions accessed by ir-SP after microinjection of CGRP was also observed when release of ir-SP was evoked by noxious mechanical stimulation of the toes. These results indicate that one important function of CGRP in the spinal cord may be the control of the intraspinal sites and neuronal circuits accessed by released substance P, possibly by inhibition of endopeptidases responsible for peptide degradation.