To determine whether IGF-I Ea, MGF, and myostatin mRNAs are related to GH-independent overload-induced muscle growth, we examined the expressions of IGF-I Ea and MGF mRNAs in the plantaris muscle after compensatory overload in hypophysectomized rats. The muscles were divided into four groups: normal-control, normal-overloaded, hypophysectomized-control, and hypophysectomized-overloaded. The weights of the plantaris muscle in the normal-overloaded were significantly higher than those of the normal-control. The weights of the hypophysectomized-overloaded were also significantly higher than those of the hypophysectomized-control. IGF-I Ea and MGF mRNAs in normal-overloaded and hypophysectomized-overloaded 3 days after overload were significantly higher than those of normal-control and hypophysectomized-control, respectively. Myostatin mRNAs in normal-overloaded and hypophysectomized-overloaded 3 days after the overload were significantly lower than those of normal-control and hypophysectomized-control, respectively. Thus, it was shown that IGF-I Ea, MGF, and myostatin mRNAs were expressed in association with muscle enlargement after compensatory overload independently of pituitary state. These observations suggest that the expression of IGF-I Ea, MGF, and myostatin mRNAs due to compensatory overload would be associated in a growth-hormone-independent manner.