Muscle fibre number and cross sectional area were studied in the response to immobilisation atrophy of the long head of the triceps brachii. Following eight weeks of immobilisation, fibre number of the muscle from the immobilised limb was compared to that of the contralateral control limb in six rats. Mean fibre cross sectional area of the LHT from the immobilised limb was compared to that of the contralateral control for another six animals. Atrophy, as estimated by a decrease in wet muscle weight, was 38.0% for the group used for fibre number estimations and 45.7% for the group used for fibre area. Fibre counts revealed no difference between muscles from immobilised and control limbs. Mean fibre area was 42.1% less for the muscle from the immobilised limb compared to the control limb. The results of this study indicate that atrophy of the LHT produced by immobilisation of the forelimb is the result of atrophy of the muscle fibres without a decrease in muscle fibre number.