It is widely recognized that prolonged increases in reactive oxygen species (ROS) and intracellular free calcium ([Ca2+]i) are part of a signaling pathway leading to cell death. ROS production resulting in oxidative stress and disruption of calcium homeostasis leading to increases in [Ca2+]i have been described as early events following exposure to a number of neurotoxicants. In order to determine the intrinsic sensitivity of developing neurons to toxicant-induced oxidative stress and disruption of calcium homeostasis, we exposed immature neurons to iron (Fe2+) or methylmercury (MeHg). Primary cultures of cortical cells (prepared from 1 day old rats) or cerebellar granule cells (prepared from 7 day old rats) were exposed to the toxicants on day in vitro (DIV) 1 (immature response to receptor agonists) or DIV 7 (mature response to receptor agonists). ROS was measured using the fluorescent probe 2',7'-dichlorodihydrofluorescin. In both cerebellar granule cells and cortical cells, Fe2+ or MeHg exposure (0.1-30 microM) produced time- and concentration-dependent increases in ROS. In general, the increase in ROS induced by both metals was greater in immature cells compared to mature cells, except for cerebellar granule cells in which the effects of Fe2+ were similar at DIV1 and 7. Changes in intracellular cation concentrations (including Ca2+) were measured using the fluorescent probe fluo-3. MeHg exposure produced a time- and concentration-dependent increase in fluo-3 fluorescence in both cerebellar granule cells and cortical cells. In cortical cultures, the fluorescence increase after MeHg exposure was greater in immature cells. In contrast, mature and immature cells were equally sensitive to the effects of MeHg in cerebellar granule cell cultures. These results suggest that there may be inherent differences in the sensitivity of neurons to toxicant-induced increases in ROS and [Ca2+] depending upon age and cell type.