Intrathecal administration of dextromethorphan and levorphanol and intravenous injection of dextromethorphan were tested on the electrophysiological response of deep multireceptive dorsal horn neurones to peripheral stimuli. Both blockade of C-fibre input to the cells and wind-up, the increase in C-fibre firing with repeated stimulus, were recorded. Intrathecal injection of levorphanol (0.25-100 micrograms) had a typical opioid effect, blocking the C-fibre input. Its affect on wind-up was dose-dependent, paralleled precisely the blocking effect on the C-fibre input and both effects were reversed by naloxone. Unlike levorphanol and other opiates, intrathecal administration of dextromethorphan (50-500 micrograms) blocked the C-fibre input and A beta response in parallel and was not reversed by naloxone. Wind-up was reduced by a maximum of 56% at the largest dose tested. Intravenous injection of dextromethorphan (5 mg/kg) also produced a reduction in wind-up but not in the C-fibre response.