Antagonists of 4 distinct regulatory sites on the N- methyl- d-aspartate (NMDA) receptor were tested for their ability to attenuate NMDA-mediated acute excitotoxicity in isolated chick retina of various embryonic ages between days 11 and 19 in ovo. Acute excitotoxicity was monitored by histology and by release of endogenous γ-aminobutyric acid (GABA) into the medium during 30 min of incubation with 50 μM NMDA. The uncompetitive PCP channel site antagonist, MK-801, the competitive antagonist, CGS 19755, and the strychnine-insensitive glycine site antagonist, 7-chlorokynurenate, completely blocked NMDA-induced cell swelling and increased GABA release at all ages tested. Potencies versus NMDA were MK-801 > CGS 19755 > 7-chlorokynurenate with IC 50s of 0.02, 0.62, and 15 μM, respectively. NMDA antagonism by the polyamine site antagonist, ifenprodil, differed from other classes of antagonists in several respects. At the earlier embryonic ages tested (E12–13) ifenprodil provided differential protection; completely blocking somal and neuritic swelling in most but not all inner nuclear layer neurons and inner plexiform processes. In dose-response studies, ifenprodil attenuated the NMDA-induced increase in medium GABA at all ages tested with an I max of 10 μM. Ifenprodil, however, showed a decreased ability to completely protect some NMDA-sensitive neurons. This was reflected both histologically and by GABA release. Maximal attenuation of NMDA evoked GABA release was 83, 80, 62 and 50% at days E12, 13, 15 and 19, respectively. Histologically, differential protection was seen at E12 and 13, in limited areas at E15, and was no longer present at E19. Prazocine, yohimbine, clonidine, BMY 14802, nimodepine and verapamil had no effect on acute NMDA-induced cell swelling or GABA release. These studies demonstrate that ifenprodil antagonism of NMDA-induced acute excitotoxicity changes with embryonic age and suggests changes in the receptor population during development.