The expression of the neurokinin 1 (NK1) receptor (i.e. substance P receptor) gene in spinal cord was studied in rats subjected to unilateral inflammation by semi-quantitative in situ hybridization analysis. Low levels of NK1 receptor mRNA were detected in many neurones throughout the grey matter. Relatively strong labelling was observed in large motoneurones and a subpopulation of superficial dorsal horn neurones. Six days after Freund's adjuvant-induced unilateral hindpaw inflammation, NK1 receptor mRNA levels in lamina I/II of the dorsal horn ipsilateral to the inflamed paw increased almost two fold compared with the contralateral side. These data suggest an inflammation-induced increase of NK1 receptor synthesis in intrinsic spinal cord neurones involved in nociceptive neurotransmission.