N-Methyl-D-aspartate (NMDA)-induced release of [3H]GABA from mouse striatal neurons in primary culture has been evaluated as a screening method for demonstrating the functional activity of potential NMDA antagonists with respect to a cellular response. Antagonists were chosen for their specificity towards each of the three principal binding sites which have been characterised on the NMDA-receptor complex: the glutamate site, the ion-channel and in particular the glycine regulatory site where several novel halogenated derivatives of kynurenic acid have been tested. All the compounds were effective in blocking [3H]GABA release and their activity was related to their potency in displacing the binding of specific ligands for each of the three sites in rat cortex membrane preparations. This was confirmed by a correlation curve for the series of kynurenate derivatives (correlation coefficient r = 0.96). The specificity of these latter compounds for the glycine site was demonstrated by the addition of excess glycine which totally reversed their inhibition but not that of antagonists acting at the glutamate or ion-channel sites. Within the kynurenate series the 5,7-dichloro derivative was shown to be more active than the 7-chloro derivative, the most active glycine antagonist previously described. These results show that this is a simple and reliable system for demonstrating a functional effect of NMDA antagonists.