Early autolytic changes in rat heart muscle were quantified using electron microscopical morphometry. Five different time groups with five rats in each group were used. All the rats were anaesthetised, and the hearts of the rats in one group were fixed by perfusion. The other rats were decapitated, and immersion fixation of the hearts performed one minute, five minutes, 10 minutes or 25 minutes after the death of the animals. When compared to perfusion fixation, the first significant changes were seen in the five minute group. The volume densities of myofibrils and glycogen decreased and the volume density of mitochondria increased. These changes were partly due to a shortening of individual sarcomeres. A gradual shortening of the sarcomeres was seen in the one, five and 10 minute groups. Distinct I bands were frequently seen only in the perfusion group. In the mitochondria a decreased matrix density and a partial disorganisation of the cristae were evident in the five minute group. Swelling of individual mitochondria was seen in the 10 minute group, but the increase in size of individual mitochondria was significant in the 25 minute group. After death, morphological changes occur very rapidly in heart muscle cells. This must be taken into consideration when, for example, slaughterhouse material is prepared and interpreted in ultrastructural studies.