Two muscle insulin-like growth factor-I (IGF-I) mRNA splice variants (IGF-IEa and IGF-IEb) have been identified in rodents. IGF-IEb, also called mechano growth factor (MGF) has been found to be up-regulated by exercise or muscle damage. Growth hormone (GH) is the principal regulator of IGF-I expression in several tissues including the skeletal muscle. Therefore, we investigated the effect of chronic GH excess or disruption of GH receptor (GHR) signaling, and acute effect of GH administration on expression of muscle IGF-I isoforms using bovine GH transgenic (bGH), GHR gene disrupted (GHR-/-) mice and GH deficient lit/lit mice before and after exogenous GH administration. MGF mRNA in skeletal muscle was increased in bGH mice whereas it was decreased in GHR-/- mice compared with control animals. Exogenous GH administration to dwarf lit/lit mice significantly increased muscle MGF but not IGF-IEa mRNA 4 h after treatment. Twelve hours after GH treatment, both MGF and IGF-IEa mRNAs in muscle were increased compared with vehicle-treated lit/lit mice. In contrast in GH sufficient lit/+ mice, both MGF and IGF-IEa mRNAs were increased 4 h after and returned to the basal level 12 h after GH treatment. Hepatic IGF-I isoforms were regulated in parallel by GH. Thus, our results demonstrated that: 1. MGF mRNA expression in skeletal muscle is paralleled with GH action; 2. MGF mRNA in muscle was produced preferentially in the situation of GH deficiency in contrast to that pattern in the GH sufficient state; 3. the induction of IGF-I isoforms by GH was tissue-specific.