Rapid Zn2+ influx through Ca2+-permeable AMPA/kainate (Ca-A/K) channels triggers reactive oxygen species (ROS) generation and is potently neurotoxic. The first aim of this study was to determine whether these effects might result from direct mitochondrial Zn2+ uptake. Adapting the mitochondrially sequestered divalent cation sensitive probe, rhod-2, to visualize mitochondrial Zn2+, present studies indicate that Zn2+ is taken up into these organelles. The specificity of the signal for Zn2+ was indicated by its reversal by Zn2+ chelation, and its mitochondrial origin indicated by its speckled extranuclear appearance and by its elimination upon pretreatment with the mitochondrial protonophore, carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP). Consistent with inhibition of mitochondrial Zn2+ uptake, FCCP also slowed the recovery of cytosolic Zn2+ elevations in Ca-A/K(+) neurons. Further studies sought clues to the high toxic potency of intracellular Zn2+. In experiments using the mitochondrial membrane polarization (DeltaPsi(m))-sensitive probe tetramethylrhodamine ethyl ester and the ROS-sensitive probe hydroethidine, brief kainate exposures in the presence of 300 microM Zn2+ (with or without Ca2+) resulted in prolonged loss of DeltaPsi(m) and corresponding prolonged ROS generation in Ca-A/K(+) neurons, in comparison to the more rapid recovery from loss of DeltaPsi(m) and transient ROS generation after kainate/1.8 mM Ca2+ exposures.