The IGF-I gene is complex and in human muscle it can be spliced into three alternative isoforms at the 3′ end: IGF-IEa, IGF-IEb, and IGF-IEc (also called MGF; Hameed et al. 2003, 2004). In rodent muscle, overexpression of the IGF-IEa or MGF results in hypertrophy (Musaro et al. 2001; Yang and Goldspink, unpublished observations). The mechanisms by which IGF-I gene expression is regulated at the muscle level may be mechanical (McCoy et al., 1999), hormonal or a combination of both. For example, in elderly subjects , recombinant human Growth Hormone (rhGH) treatment increases the expression of IGF-IEa, but when combined with exercise markedly increases the MGF isoform (Hameed et al, 2004). It has recently been shown that muscle cells grown in 3-D collagen matrixes upregulate IGF-I transcript expression in response to stretching of the matrix (Cheema et al., 2004). In the present work we have studied muscle cells in culture with the aim of determining if either or both of the two splice variants of IGF-I would be upregulated when treated with GH and/or IGF-I, in the absence of mechanical signals C2C12 myoblasts were grown to 50% confluency in medium containing 10% foetal calf serum (FCS). The cells were transferred to medium containing i) 1%FCS only, ii) 100 ng/ml rhGH iii) 100 pg/ml IGF or iv) both. In the untreated cells (i) both isoforms (IGF-IEa and MGF) were present. Treatment with rhGH alone lead to an increase in IGF-IEa and MGF of about 3 fold over control (Table 1). However, treatment with IGF-I abolished expression of both isoforms. When used in combination, the inhibitory effect of IGF-I overode the GH stimulation of IGFIEa and MGF transcription. We conclude that muscle tissue can upregulate IGF-I isoform expression as a direct result of hormonal stimulation or stretch stimuli. The isoforms of IGF-I seem equally sensitive to GH stimulation in vitro.In vivo, a negative feedback mechanism may modulate the action of GH on IGF-I transcription in muscle tissue in by circulating or local IGF-I expression.