[Show abstract][Hide abstract] ABSTRACT: The purpose of this research is to find some useful spectroscopic factors in human tear fluid contents to monitor diurnal changes of the physicochemical ocular conditions noninvasively. All tear fluid samples were collected with glass microcapillary tubes from both eyes of three donors and analyzed by Fourier transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR). We measured the peak intensities at 2852, 1735, 1546, and 1242 cm(-1), and the peak intensity ratios among those peaks in the second derivative spectra. We found significant diurnal and individual variations in those peak intensities for tear fluid obtained from right and left eyes. Among these variations, we observed significant changes in tear samples between right and left eyes. In this case the peak intensity ratio between 1242 (phosphate ester) and 2852 cm(-1) (fatty acid methylene) of right eye tear fluid was increased in the afternoon (1600 to 1900 h), while that of left eye tear fluid did not change significantly. In the ratio between 1242 (phosphate ester) and 1546 cm(-1) (amide II), the difference was not observed between both eyes. We conclude that the difference in diurnal variations of biochemical constituents between right and left eye tear fluids could be monitored noninvasively and nondestructively by FTIR technique and this method could be useful in the future for tear diagnoses.