Denervation of the anterior latissimus dorsi (ALD) muscle causes transient muscle fibre hypertrophy and leads to an increase of the SM1 myosin isoform. We tested whether the changes that take place after denervation can be attributed to loss of muscle activity which follows denervation. Neuromuscular activity was prevented by blocking the acetylcholine receptors with alpha-bungarotoxin and thereby paralysing the muscle. Following this treatment, we found increased muscle weight and pronounced hypertrophy of muscle fibres. Also, the proportion of SM1 isomyosin was decreased. Due to the multiple innervation of ALD muscle fibres it is possible to paralyze only part of the muscle. When only a region of the muscle was paralysed a local hypertrophy of fibres was detected, and the change from SM1 to SM2 was most pronounced in the area where activity was blocked. Removal of muscle activity resulted in changes similar to those that occurred after denervation.