The nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), administered i.v. (50 mg kg-1) or by iontophoresis, was tested on the responses of spinal dorsal horn neurones in cats anaesthetized with alpha-chloralose and spinally transected at the L1 level. Extracellular, single-unit recordings were obtained from functionally identified dorsal horn cells. All units included in this study were wide dynamic range neurones. L-NAME significantly reduced the responses of (i) twelve neurones to noxious thermal stimulation of the receptive field, (ii) nine neurones to noxious pinch, (iii) nine neurones to iontophoretic application of N-methyl-D-aspartate (NMDA) and (iv) ten neurones to iontophoretic application of substance P. The inhibition usually lasted for 50-70 min following i.v. administration and for 5-8 min after iontophoretic application of L-NAME. The responses of four neurones to iontophoretic application of quisqualate were not affected by L-NAME. The results suggest the possible involvement of nitric oxide in the mediation of the spinal effects of NMDA and substance P, and in the transmission of thermal and mechanical nociceptive imputs.