Human muscle cell growth was studied to investigate the nucleo-cytoplasmic ratio during development and ageing. The mean muscle fibre size, as well as the number and size of myonuclei were estimated in human muscle biopsies from normal individuals whose ages ranged from 1 to 71 years. Changes in muscle fibre size were accompanied by similar changes in nuclear number, the size of which remained the same. This resulted in a constant nucleo-cytoplasmic ratio during the age range studied. The role of the satellite cell in the maintenance of this ratio was discussed.