The enantiomers of the potent non-competitive NMDA receptor antagonist ketamine and its major metabolite, norketamine were evaluated as NMDA receptor antagonists using the rat cortical wedge preparation and the neonatal rat spinal cord preparation, respectively, for electrophysiological studies and [3H](RS)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-im ine ([3H]MK801) in homogenate binding experiments. In agreement with earlier studies (S)-ketamine (Ki 0.3 microM) was found to possess a 5 times higher affinity for the NMDA receptor complex than (R)-ketamine (Ki 1.4 microM). (S)-Norketamine (Ki 1.7 microM) had approximately an 8 times higher affinity than (R)-norketamine (Ki 13 microM) in the inhibition of [3H]MK-801 binding. All compounds inhibited responses to NMDA in the rat cortical wedge preparation and the hemisected neonatal rat spinal cord, being approximately four times more potent in the cortex than in the spinal cord except for (R)-norketamine being only twice as potent. In light of the clinically obtained concentrations of norketamine after oral administration of ketamine, these data strongly suggest that (S)-norketamine may contribute significantly to the clinical activity of (S)-ketamine, especially when given orally.