The opiate agonists, ketobemidone, methadone and pethidine, were evaluated as N-methyl-D-aspartate (NMDA) receptor antagonists using the rat cortical wedge preparation and the neonatal rat spinal cord preparation for electrophysiological studies and [3H](RS)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-im ine ([3H]MK-801) binding experiments using rat forebrain homogenates. Ketobemidone, methadone and pethidine were inhibitors of [3H]MK-801 binding with Ki values of 26 microM, 0.85 microM and 47 microM, respectively. In the cortex, 1 mM ketobemidone and 1 mM methadone reduced NMDA responses, but not (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl) propionic acid (AMPA) or kainate responses in an use-dependent manner, whereas 1 mM pethidine was devoid of antagonist activity. In the spinal cord preparation, the activities of ketobemidone and methadone were weaker than in cortex. In contrast, pethidine was equipotent with ketobemidone in the spinal cord. These results suggest that ketobemidone and methadone may be useful therapeutic agents in conditions where a combined opiate agonist and NMDA antagonist treatment is desired.