Occupational and environmental exposure to increased concentrations of manganese (Mn) can lead to an accumulation of this element in the brain. The consequence is an irreversible damage of dopaminergic neurons leading to a disease called manganism with a clinical presentation similar to the one observed in Parkinson's disease. Human as well as animal studies indicate that Mn is mainly bound to low molecular mass (LMM) compounds such as Mn-citrate when crossing neural barriers. The shift toward LMM compounds might already take place in serum due to elevated Mn concentrations in the body. In this study, we investigated Mn-species pattern in serum in two different animal models by size exclusion chromatography-inductively coupled plasma mass spectrometry (SEC-ICP-MS). A subchronic feeding of rats with elevated levels of Mn led to an increase in LMM compounds, mainly Mn-citrate and Mn bound to amino acids. In addition, a single i.v. injection of Mn showed an increase in Mn-transferrin and Mn bound to amino acids 1 h after injection, while species values were more or less rebalanced 4 days after the injection. Results from Mn-speciation were correlated to the brain metabolome determined by means of electrospray ionization ion cyclotron resonance Fourier transform mass spectrometry (ESI-ICR/FT-MS). The powerful combination of Mn-speciation in serum with metabolomics of the brain underlined the need for Mn-speciation in exposure scenarios instead of the determination of whole Mn concentrations in blood. The progress of Mn-induced neuronal injury might therefore be assessed on the basis of known serum Mn-species.