In the present study, we analyze the effect of Fe2+/ascorbate-induced lipid peroxidation on Ca(2+)-dependent and Ca(2+)-independent release and on the uptake of gamma-[3H]aminobutyric acid (GABA) by sheep brain synaptosomes. In addition, we study the effect of lipid peroxidation on the levels of cytosolic calcium and on the uptake of calcium (45Ca2+). After membrane lipid peroxidation, a decrease in the uptake of GABA is observed. After ascorbate/Fe(2+)-induced membrane lipid peroxidation, a significant decrease in [3H]GABA release in response to K(+)-depolarization occurs, in the absence and in the presence of Ca2+. The influx of 45Ca2+ induced by K(+)-depolarization is significantly depressed under peroxidative conditions, while basal calcium uptake is inhibited to a much lesser degree. The levels of free ionic calcium [Ca2+]i, as determined by the fluorescent dye Indo-1, are increased after synaptosomes were submitted to the ascorbate/Fe2+ oxidative stress. It is concluded that membrane lipid peroxidation induces a decrease in Ca(2+)-dependent and Ca(2+)-independent efflux of accumulated [3H]GABA in response to elevated K+ pulses (60 mM) and in the depolarization-induced calcium influx, while free ionic calcium levels increase. The Ca(2+)-dependent efflux is interpreted to reflect stimulus-secretion coupling process and the Ca(2+)-independent efflux may reflect membrane transport processes. Thus, the results suggest a possible relationship between a reduced calcium movement across the membrane, the decrease in neurotransmitters uptake and release and oxidative stress.