Calcium influx via the NMDA receptor has been proposed as a mechanism of hypoxia-induced neuronal injury. The present study tests the hypothesis that the increase of [Ca2+]i observed under hypoxic conditions is the result of an NMDA-mediated Ca2+ influx. Changes of [Ca2+]i, measured fluorometrically with Fura-2, were followed after activation of the NMDA receptor with NMDA and glutamate, in the presence of glycine, in cortical synaptosomes prepared from six normoxic and six hypoxic guinea pig fetuses. [Ca2+]i was significantly higher in hypoxic vs normoxic synaptosomes, at baseline and in the presence of glycine as well as following activation of the NMDA receptor. Increase in [Ca2+]i was not observed in a Ca2+ free medium and was significantly decreased by MK-801 and thapsigargin. These results demonstrate that hypoxia-induced modifications of the NMDA receptor ion-channel results in increased [Ca2+]i in hypoxic vs normoxic synaptosomes. This increased accumulation may be due to an initial influx of Ca2+ via the altered NMDA receptor with subsequent release of Ca2+ from intracellular stores. Increase in intracellular calcium may initiate several pathways of free radical generation including cyclooxygenase, lipoxygenase, xanthine oxidase and nitric oxide synthase, and lead to membrane lipid peroxidation resulting in neuronal cell damage.