Hearts of rats aged 26-27 months (7 animals) and 4 months (16 animals) were examined. The number of muscle fibres/mm2 in the heart was the same at both ages. In old hearts more areas with fibre atrophy or hypertrophy were found. Capillary density was decreased (p<0.01) in old rats and the diffusion distance was increased (p<0.01). The F/C R index was also increased (p<0.01). In the old rats no correlation could be demonstrated between the diffusion distance and the weight of the heart which is regularly demonstrable in young animals.