Parkinson's disease is associated with abnormal, aggregated forms of the protein, a-synuclein. We investigated the effects of calcium on a-synuclein aggregation. Incubation of monomeric a-synuclein (24 h) at low concentration (10 lM) with calcium resulted in surface aggregates (1.5 ᠰ.7 lm2) detected by fluorescence microscopy, saturating at a half-maximum calcium concentration of 80 lM, whilst incubations without calcium showed few aggregates. Scanning electron microscopy revealed that surface arrays of 10-20 nm globular a-synuclein particles formed in the presence of calcium. Incubation of a-synuclein at high concentration (75 lM; 6 h) resulted in soluble oligomeric aggregates as detected by fluorescence correlation spectroscopy in a calcium dependent process, saturating at a half-maximum calcium concentration of 180 lM. In cell culture experiments, we used thapsigargin or calcium ionophore A23187 to induce transient increases in intracellular free calcium in human 1321N1 cells expressing asynuclein- GFP and observed calcium flux and a-synuclein aggregation by fluorescence microscopy. The cell culture data shows that a transient increase in intracellular free calcium (2-3 h) significantly increased the proportion of cells bearing cytoplasmic a-synuclein aggregates at 6 & 12 h post-treatment (p < 0.01). Our data indicates that calcium promotes a-synuclein aggregation both in vitro and in the cellular environment and suggests that surface adsorption may play an important role in the calcium-dependent aggregation mechanism.