C³H/HeJ (C³H) mice exhibit 30-40% higher serum IGF-I than do C57BL/6J (B6) mice, in association with increased bone mineral density and strength. These differences are inherited, and thus provide a model for determining molecular mechanisms for genetic variation of serum IGF-I and downstream actions. We now report that increased serum IGF-I in C³H mice is associated with increased transcription from the minor exon 2 promoter in liver from female and male mice. The increase in hepatic IGF-I gene expression caused by increased abundance of IGF-I mRNA transcribed from the exon 2 promoter can quantitatively account for the increased serum IGF-I in C³H mice. Also, levels of both Ea and Eb IGF-I mRNAs are increased in livers of male C³H mice. Fasting lowered serum IGF-I and liver IGF-I mRNA levels in female mice of both strains. However, serum IGF-I and liver IGF-I mRNA levels remained higher in fasted C³H mice compared with fasted B6 mice. Levels of IGF-I transcripts initiated from exon 2 are also significantly increased in skeletal muscle, fat, ovaries and kidneys of C³H mice. IGFBP-5 mRNA levels are significantly higher in muscle and fat of C³H mice than in B6 mice. Levels of exon 1-containing transcripts are increased in whole femurs of male and female C³H mice. We conclude that increased transcription of the IGF-I gene occurs in a promoter- and tissue-specific manner in C³H mice. The increased IGFBP-5 mRNA levels in fat and muscle suggest that IGF-I signaling is increased in these tissues in C³H mice.