Aims: To determine whether sex differences exist in tissue oxygen saturation (StO2) and the hemoglobin (Hb) oxygenation state of the resting human masseter muscle. Methods: Near-infrared spectroscopy (NIRS) was used to measure StO2 and Hb oxygenation state in 20 healthy adult volunteers (10 women and 10 men). To determine the measurement range and reliability of the NIRS recording probe, the probe was set up on 12 layers of white acrylic resin plate, each 3 mm thick. Total hemoglobin levels were measured while a red vinyl resin plate, 1 mm thick, was inserted in turn between each of the 12 layers. Distances from the skin surface to the lateral surface (S-L) and to the medial surface (S-M) of the right masseter at the middle portion of the masseter were measured on T1- weighted magnetic resonance images (repetition time 500 ms, echo time 23 ms). Thickness of the masseter was calculated by subtraction [(S-M) - (S-L)]. For the study of Hb oxygenation state, the probe was positioned at the same position on the skin surface at the mandibular postural (rest) position. Results: The measurement range of the NIRS probe was from 9 to 21 mm under the skin, and the reliability of the probe was judged by intra- and inter-class correlation coefficients. There was no sex difference in S-L and the thickness of the masseter; the means of S-L and masseter thickness were 9.3 mm and 15.5 mm in men and 9.8 mm and 14.3 mm in women, respectively. Except for StO2 values, there were significant sex differences in the Hb oxygenation parameters, with the mean values in the men being approximately twice those in the women. Conclusion: These results provide evidence that a sex difference in the Hb oxygenation state may exist in the masseter muscle of normal healthy subjects. Schlagwörter: hemoglobin oxygenation, hemodynamics, blood flow, masseter muscle