State III (activated) mitochondrial respiration rates with pyruvate/malate, glutamate/malate, and succinate as substrates were assayed in isolated intact skeletal muscle mitochondria in 29 subjects aged 16-92 years. There was a significant negative correlation between respiration rate and age with all substrates tested. A similar trend was seen for respiratory enzyme activities assayed in muscle homogenate. These findings suggest a substantial fall in mitochondrial oxidative capacity in ageing muscle, which may contribute to reduced exercise capacity in elderly people. Mitochondrial respiratory failure may contribute to the ageing process in other organs.