Cell loss in Parkinson's and Parkinson's-plus diseases is linked to abnormal, aggregated forms of the cytoplasmic protein, a-synuclein (a-syn). The factors causing a-syn aggregation may include oxidative stress, changes in protein turnover and dysregulation of calcium homeostasis, resulting in cytotoxic aggregated a-syn species. Recently, we showed that raised calcium can promote a-syn aggregation. We have now investigated the effects of raised calcium combined with oxidation/oxidative stress on a-syn aggregation both in vitro and in vivo. We treated monomeric a-syn with calcium, hydrogen peroxide or calcium plus hydrogen peroxide in vitro and used size exclusion chromatography, fluorescence correlation spectroscopy, atomic force microscopy and scanning electron microscopy to investigate protein aggregation. Our in vitro data is consistent with a cooperative interaction between calcium and oxidation resulting in a-syn oligomers. In cell culture experiments, we used thapsigargin or ionophore A23187 to induce transient increases of intracellular free calcium in human 1321N1 cells expressing an a-syn-GFP construct both with and without co-treatment with hydrogen peroxide and observed a-syn aggregation by fluorescence microscopy. Our in vivo cell culture data shows that either transient increase in intracellular free calcium or hydrogen peroxide treatment individually were able to induce significantly (P=0.01) increased 1-4孠cytoplasmic a-syn aggregates after 12h in cells transiently transfected with a-syn-GFP. There was a greater proportion of cells positive for aggregates when both raised calcium and oxidative stress were combined, with a significantly increased proportion (P=0.001) of cells with multiple (3 or more) discrete a-syn focal accumulations per cell in the combined treatment compared to raised calcium only. Our data indicates that calcium and oxidation/oxidative stress can cooperatively promote a-syn aggregation both in vitro and in vivo and suggests that oxidative stress may play an important role in the calcium-dependent aggregation mechanism.