The effect of ageing on voluntary and electrically evoked muscle force was investigated. A group of 70-year-old men and a control group of young men took part in the study. The muscles studied were the elbow flexor group and the triceps surae group of the leg. With ageing both muscle groups showed a reduction in maximum voluntary force and in the force of electrically evoked tetani. The reduction was greatest in the leg muscles. The time to peak of the twitch was prolonged with age in the leg muscles. The findings are discussed in relation to the changes in fibre type known to occur in elderly muscle.