Exposure of rat retinal cultures to HIV-1 coat protein gp120 for several minutes increases [Ca’+] i in approximately half of the ganglion cells; this effect is associated with delayed-onset neuronal injury, similar to that previously reported in NMDA receptor-mediated neurotoxicity. Here we show that NMDA antagonists can prevent both the rise in [Ca*+] i and subsequent neuronal damage engendered by 20 pM gp120. However, wholecell patch-clamp recordings demonstrate that gpl20 does not directly evoke an NMDA-like response or enhance glutamate/NMDA-activated currents. Moreover, complete protection from gpl204nduced [Ca*+] i increases and neurotoxicity is afforded by incubation with glutamate-pyruvate transaminase, which breaks down endogenous glutamate as verified by HPLC. Since, under standard conditions in these cultures, neither glutamate nor a low picomolar concentration of gp120 is deleterious on its own, our results suggest that their neurotoxicity is synergistic.