Glutamate is an excitotoxin responsible for causing neuronal damage associated with mitochondria dysfunction. We have analyzed the relationship between the mitochondrial respiratory rate, the membrane potential (delta psi) and the activity of mitochondrial complexes in retinal cells in culture, used as neuronal models. Glutamate (10 microM-10 mM) dose-dependently decreased the O2 consumption and the membrane potential. A linear correlation was found between these parameters, suggesting that the mitochondrial respiratory function was affected. Exposure to glutamate (100 microM) for 10 min, in the absence of Mg2+, inhibited the activity of complex I (26.3%), complexes II/III (22.2%) and complex IV (26.7%). MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine hydrogen maleate), a non-competitive antagonist of the NMDA (N-methyl-D-aspartate) receptors, completely reversed the effect exerted by 100 microM glutamate at the level of complexes I, II/III and IV. These results suggest that NMDA receptor-mediated inhibition of mitochondrial respiratory chain complexes may be responsible for the alteration in the respiratory rate of chick retinal cells submitted to glutamate.