Alertness is a basic psychological function within the construct of attention. The neural basis of alertness has been found in the right frontotemporal cortex and the left ventrolateral prefrontal cortex. The aim of the present study was to establish the measurement of functional brain activity during alertness with functional near infrared spectroscopy (fNIRS). Therefore we measured the changes in brain oxygenation in two independent samples during an alertness condition and control conditions with 52 NIRS channels placed over the frontotemporal area. Based on a region of interest defined in the first sample (and on the literature) we showed in the second sample significantly higher activation during the alertness condition compared to the control conditions. This study proves the possibility of investigating the neural correlates of alertness using fNIRS.