The D-enantiomer of bromohomoibotenic acid (Br-HIBO) was inactive in electrophysiological experiments when administered alone, but enhanced depolarizations evoked by L-Br-HIBO or quisqualate when co-administered with these agonists. In addition, quisqualate induced a long-lasting (> 120 min) sensitization of cortical wedge neurons to D-Br-HIBO. This latter effect of D-Br-HIBO was similar to, but significantly more potent and selective, than the earlier observed quisqualate-induced sensitization of cortical neurones to depolarization by (S)-2-amino-4-phosphonobutyric acid (L-AP4).