The Leu-enkephalin dalargin does not normally penetrate the blood brain barrier when given intravenously. Drug targeting to the brain was investigated by using poly(butylcyanoacrylate) nanoparticles which were coated with polysorbate 80. When injected intravenously in mice, dalargin-loaded nanoparticles coated with the polysorbate 80 induced an analgesic effect at doses of 5.0 mg/kg and 7.5 mg/kg dalargin as shown by hindlimb licking on the hot plate. Neither the intravenous injection of dalargin alone at various doses nor the mixture of dalargin-loaded nanoparticles without the polysorbate 80 were able to induce an analgesic activity. This confirms previous observations that nanoparticles provide a convenient method to deliver drugs across the blood-brain barrier.