['To apply Fourier transform infrared (FTIR) spectroscopy to study the process of postmortem degradation of the rat brain and to provide a new way for the estimation of postmortem interval (PMI).', 'The rats were sacrificed by cervical dislocation and the bodies were kept in a controlled environmental chamber set at (30 +/- 2) degrees C. To measure the content of the chemical groups in postmortem rat brains at the different time points from 0 to 36 h using the FTIR spectrograph.', 'With prolongation of PMI, the peak position of main absorbance bands in the FTIR spectra showed no significant changes, while the peak levels showed dramatic changes: (1) The relative peak intensity of 1080 cm(-1), 1238 cm(-1) (I1080/I1398, I1238/I1398) associated with nucleic acid decreased obviously; (2) The peak intensity ratio at Amide I, II (I1647/I1541) decreased; (3) The peak intensities at 1456 cm(-1) and 1398 cm(-1) showed a decreased and an increased trend, respectively; (4) Compared to the peak intensity of 1647 cm(-1), the peak intensities at 2852 cm(-1), 2871 cm(-1), 2923 cm(-1), and 2958 cm(-1) tended to increase, with only a slightly increased tendency in peak intensity of 2871 cm(-1).'] Do [ The changes of Fourier transform infrared spectroscopy in rat 's brain ]?