The antagonistic action of D-baclofen at baclofen receptors mediating antinociception in the spinal cord was examined. Drugs were administered intrathecally to rats and effects on nociceptive threshold evaluated in the tail flick test. L-Baclofen, D-baclofen and the racemate produced dose-related increases in tail flick latency, with L-baclofen being twice as potent as the racemate and approximately 100 times more potent than D-baclofen. When D-baclofen was injected 15 min prior to L-baclofen, it produced a dose-related inhibition of the effect of L-baclofen. Concomitant administration produced a more ambiguous effect. Antagonism appeared specific for baclofen receptors because analogues with full and partial agonist activity as well as an agonist dose of D-baclofen, but not morphine or noradrenaline, were inhibited by pretreatment with D-baclofen. gamma-Aminobutyric acid (GABA) did not increase tail flick latency either alone or following pretreatment with an uptake inhibitor or a GABA-transaminase inhibitor. Antinociception produced by intrathecal administration of Baclofen appears to result from activation of a receptor which is stereoselective for the L-isomer and can be blocked by D-baclofen in doses which have initial agonist activity. This receptor may not be a GABA subtype because GABA does not mimic the effect of baclofen and the rank order of potency of analogues differs from established GABAB systems.