The presence and cellular localization of insulin-like growth factor-I (IGF-I) mRNA in the small intestine of suckling and adult rats was studied. A sensitive reverse transcription (RT) competitive-polymerase chain reaction (PCR) revealed IGF-I gene expression in both age groups. Adult tissue contained 3-fold higher levels of IGF-I mRNA in comparison with sucklings. Using an in situ hybridization technique, IGF-I transcripts were localized mainly in enterocytes and goblet cells in the intestinal crypts of adult rats. By using this technique, IGF-I mRNA was not detected in jejunum of 12-day-old rats.