Growth factors play an important role in the regulation of the growth and development of many tissues including muscle. Muscle is derived from somitic cells that differentiate to form myoblasts. These myoblasts then fuse to form myofibres. As these myofibres mature they take on the morphological, biochemical and physiological properties of mature muscle fibres. It is the plasticity of muscle fibres with respect to these properties that gives skeletal muscle its remarkable ability to adapt to different conditions of loading and usage. The aim of this thesis was to determine the role of growth factors in regulating muscle fibre type and fibre size in two different models of altered muscle mass. Bovine muscle development was examined in normal foetuses (NM) and in double-muscled Belgian Blue foetuses (DM) carrying a mutation in the gene for myostatin. Muscle fibre type and fibre size were characterised using semi-automated computer-assisted image analysis after myofibrillar ATPase histochemical staining. Plasma IGF levels were measured, IGF-1 mRNA was measured using in situ hybridisation and IGF receptor binding was analysed by receptor autoradiography. Myostatin mRNA was analysed using semi-quantitative RT-PCR. Skeletal muscle hypertrophy and atrophy were induced m adult rabbits by immobilisation of the hind limb with muscles in a lengthened and shortened position respectively. Fibre type and fibre size changes were analysed as described above and myostatin mRNA expression levels were measured using Northern blot analysis and semi-quantitative RT-PCR. Results showed there was a biphasic change in the percentage of type 1 muscle fibres in both NM and DM, with the percentage initially declining, then increasing again. The percentage of type 1 fibres was consistently lower in DM. Circulating levels of IGF-1 increased with gestational age and levels of IGF-1 and IGF-2 were not different between NM and DM. IGF-1 mRNA expression in skeletal muscle declined with age in a pattern that was unrelated to changes in fibre type. Levels of IGF-1 mRNA were lower in DM during late gestation and this decrease was temporally linked to a reduced area of type 1 fibres in DM at this stage of development. Myostatin mRNA declined with gestational age, and levels of the mutant mRNA were higher in DM. In induced hypertrophy, myostatin mRNA expression was lower than in controls and in atrophy, myostatin mRNA was elevated at 2 days but not at 6 days. There were no changes in fibre type or fibre size over the treatment period. Myostatin protein was evenly localised to all muscle fibre types. In conclusion, IGF-1 appears to be unrelated to changes in fibre type during bovine muscle development, but may play a role in the regulation of fibre size. The DM condition is associated with a change in fibre type, but the role of myostatin in mediating this change remains unknown. Changes in myostatin mRNA in response to induced hypertrophy and atrophy strongly suggest an important role for myostatin as a negative regulator of skeletal muscle mass in postnatal animals.