The effects of novel substance P (NK-1) receptor antagonists LY303870 and LY306740, as well as LY306155, the enantiomer of LY303870, were tested on the responses of nociceptive spinal dorsal horn neurons to iontophoretically applied substance P and to peripheral noxious stimuli. The peripheral stimuli included noxious thermal and pinch stimuli applied to the cutaneous receptive field in the hind paw and stimulation of the superficial peroneal nerve with a train of high-intensity electrical stimuli. Extracellular recordings were obtained using multi-barrel electrodes from L4-L7 segments of the spinal cord in cats anaesthetized with alpha-chloralose and spinalized at the L1 level. The antagonists were given i.v. (0.5-4.0 mg/kg). Responses to substance P were inhibited by LY303870 and by LY306740 in a dose-related manner, but were not affected by LY306155. Responses to peripheral noxious thermal stimulation were inhibited in a dose-related manner by LY303870 and LY306740, and only at higher doses (2 mg/kg or more) by LY306155. Responses to pinch stimuli were inhibited by LY303870 and LY306740. LY306155 lacked consistent effects on pinch responses. LY303870 selectively inhibited the late component of the response to electrical stimulation of the superficial peroneal nerve. When these three drugs were tested against the responses of dorsal horn neurons to the excitatory amino acid, N-methyl-D-aspartate, the responses were unaffected. These data suggest that LY303870 and LY306740 pass from the circulation into the spinal cord where they antagonize dorsal horn neuronal responses to substance P and nociceptive inputs.