The increasing incidence of neurodegenerative and other diseases is considered to involve an excessive production of reactive oxygen species (ROS). Water supplies are often characterized by excessive organic waste that is decomposed by bacteria, using dissolved oxygen, leading to oxygen depletion. The potassium content of these waters may also affect negatively the mitochondrial metabolism and cellular ROS formation. This work focused on characterizing mitochondrial autofluorescence changes, with flavoprotein origin, and fluorescence ROS signals measured using the 2',7'-dichlorodihydrofluorescein diacetate indicator H2DCFDA. All signals were evoked by hypoxia or by the depolarizing agent KCl (20 mM), at the hippocampal mossy fiber synapses of CA3 area. It was observed that both hypoxia and KCl-induced depolarization elicited a small rise in the autofluorescence and ROS changes. The hypoxia-induced signals were maintained upon normal reoxygenation, but of those evoked by KCl, the autofluorescence signals recovered during washout, while the ROS changes were irreversible.