Manganese is an essential metal in human that functions in many enzymes. In contrast excessive exposure to Mn results in neurotoxicity. Accumulation of manganese damages central nervous system and causes Parkinson disease like syndrome called manganism. Mn neurotoxicity has been suggested to involve an imbalance in catecholamine neurotransmitters. It hypothesized that Mn can obstruct catecholamine synthesis pathway by inhibition of Tyrosine hydroxylase. Previous studies demonstrated that chronic and acute dose of Mn has different possible effects on catecholamine synthesis. It’s assumed that an acute dose of manganese can kill dopaminergic cells. Therefore, we focused the effect of Mn in catecholamine concentration on the rat’s brain by MnCl2 injection intraperitoneally and analyzed their brains after the time interval. This study used different acute doses in short time courses and different chronic doses at different exposing times to investigate which of them (exposing dose or time) is more important in Mn toxic effect. Measurement of catecholamine concentration performed by fluorescence assay in acidic and oxidant environment.