The effect of intrathecal (i.t.) injection of the analgesic agents, codeine, buprenorphine, tilidine and one of its metabolites, nortilidine, tramadol and nefopam, was determined in the tail-flick test performed on rats. ED50 values were derived from the dose-response lines. The relative potency ranking established from the ED50 values is buprenorphine (0.4 nM) greater than nortilidine (29 nM) = tramadol (26 nM) = nefopam (34 nM) greater than codeine (42 nM) greater than tilidine (118 nM). An i.t. injection of the opiate antagonist, naloxone (5 micrograms), prevented the antinociceptive effect of all analgesic agents administered at the highest dose tested. It is concluded that these analgesic agents, like morphine, exert their effect at least in part through a spinal site of action.