1. Muscle fibres may be subdivided into type I (with slow-twitch contractile properties) and type II (fast-twitch) depending on their myosin adenosine triphosphatase activity. In voluntary isometric contractions type I fibres are utilized at low forces (less than 20% of maximum) whereas type II fibres are recruited in addition at high forces. This physiological recruitment order has enabled us to measure the relaxtion rate of type I and II fibres in vivo in normal human subjects. 2. Relaxation rate was measured in 16 subjects from low (10% of maximum) and maximum isometric quadriceps contractions and the muscle-fibre type composition determined from needle-biopsy specimens in 10 subjects. The relaxation rate of type II fibres was calculated to be twice as fast as that of type I. 3. It was not possible to estimate, from studies in 33 quadriceps muscles (25 normal subjects), the contribution of type II fibres to overall fibre area from the relaxation rate as determined from electrically stimulated isometric contractions.