The aim of this study was to observe the morphological changes of muscle in the process of rigor mor-tis. The quadriceps of 40 rats at various postmortem intervals were observed under the scanning electron microscope (SEM) and the light microscope by phosphotungstic acid-haematoxylin (PTAH) stain. The re-sults showed that the striations of muscle were blurred within 4h, but they became apparent from 6h to 24h after death. The authors suggest that this phenomenon be associated with the increased resistance of muscle against the postmrtal changes. THe observations by scanning electron microscopy and light mi-croscopy have revealed that the muscles do eontract in the process of rigor mortis because the distance be-tween two Z lines shortens and the I band narrows, compared with those in anaesthetised animals. The ba-sic biochemical process for the formation of rigor mortis is the same as that of muscle contraction except that the former happens postmortem and the latter antemortem.