The sternomastoid, biceps brachii and tibialis anterior muscles of ageing mice were compared with controls. The biceps brachii and tibialis anterior were lighter in the ageing mice and contained fewer fibres. There was also evidence of compensatory hypertrophy present. The mean fibre lengths of the three muscles decreased with age as a result of significant reductions in the number of sarcomeres along the fibres. Sarcomere length remained unaltered. The reduction in the number of sarcomeres along the fibres may be due to an inability of ageing muscle to respond to the tension acting on it. As fibre length determines the range of movement at a joint, the decrease may also contribute to the reduced mobility commonly seen in old age.