Solving arithmetical problems is a core skill which is learned starting early in childhood and has been shown to involve a temporo-parietal network. In this study, we investigated hemodynamic concentration changes in oxygenated (O(2)Hb) and deoxygenated hemoglobin (HHb) within cortical brain regions by means of near-infrared spectroscopy (NIRS). Ten healthy subjects had to calculate or just read two-digit addition tasks that were either presented as numeric formulas or embedded in text. We found higher increases for O(2)Hb in parietal brain regions of both hemispheres for the calculation compared to the reading-only condition. Furthermore, these increases were more pronounced during text-embedded tasks than during numeric tasks. Corresponding decreases of HHb could also be detected. These first NIRS findings on that topic confirm that parietal regions are involved in the processing of arithmetic tasks while the amount of activation seems to depend on task modalities like difficulty or complexity.