The activity of single thalamic nociceptive neurones was recorded extracellularly. BW 180C, (D-ala2,D-leu5-enkephalin) applied locally, by iontophoresis (10--40 nA) or systemically by i.v. injection (1.5--6 mg/kg) markedly depressed the evoked excitation of these neurones following peripheral noxious stimulation. This effect could be antagonised by naloxone. Enhanced enzymic stability did not appear to prolong the duration of action of BW 180C iontrophoretically applied to these neurones.