Nanoscale zero-valent iron (nZVI) is a “redox”-active nanomaterial used in the remediation of contaminated groundwater. To assess the effect of “aging” and surface modification on its potential neurotoxicity, cultured rodent microglia (BV2) and neurons (N27) were exposed to fresh nZVI, “aged” (>11 months) nZVI, magnetite, and polyaspartate surface-modified (SM) nZVI. Increases in various measures of oxidative stress indicated that BV2 microglia responded to these materials in the following rank order: nZVI > “aged” nZVI > magnetite = SM nZVI. Fresh nZVI produced morphological evidence of mitochondrial swelling and apoptosis. In N27 neurons, ATP levels were reduced in the following rank order: nZVI > SM-nZVI >“aged” nZVI = magnetite. Ultrastructurally, nZVI produced a perinuclear floccular material and cytoplasmic granularity. Both SM-nZVI produced intracellular deposits of nanosize particles in the N27. The physicochemical properties of each material, measured under exposure conditions, indicated that all had electronegative zeta potentials. The iron content of nZVI (35%) and SM-nZVI (25%) indicated high “redox” activity while that of “aged” and magnetite was neglibile. Sedimentation and agglomeration occurred in the following rank order: nZV > “aged” nZVI > magnetite SM-nZVI. Correlating these properties with toxicity indicated that partial or complete oxidation of nZVI reduced its “redox” activity, agglomeration, sedimentation rate, and toxicity to mammalian cells. Surface modification decreased nZVI toxicity by reducing sedimentation which limited particle exposure to the cells.