The expression and localization of insulin-like growth factor-I (IGF-I) in the four parts (tip, upper, mid and base) of the red deer antler has been extensively investigated. We used reverse transcriptase polymerase chain reaction (RT-PCR) and real-time reverse transcriptase polymerase chain reaction (real time RT-PCR), in situ hybridization, immunohistochemistry and Western blot techniques to localize IGF-I messenger ribonucleic acid (mRNA) and IGF-I peptide in the four parts of the antler. The specific sequence encoding IGF-I was detected by RT-PCR in all of the four specimens, and the 395 bp IGF-I sequence from the red deer antler was shown to have very high homology with human, goat and mouse IGF-I. In situ hybridization and immunohistochemistry results demonstrated that the expression of IGF-I occurred in chondrocytes and osteoblasts in the tip and upper parts of the antler. However, IGF-I was only detectable in osteoblasts around the bone in the mid and base parts. There were significant differences in the intensity of the signal obtained with the IGF-I probe in the tip, upper, mid and base tissues. The Western blot analysis also provided evidence that IGF-I expression was localized differentially in the four parts of the deer antler. This study indicates that antler tissue is an essential part of the IGF system, which is involved in the regulation of the growth of red deer antlers. The specific expression of IGF-I in the four parts of the deer antler suggests that the IGF-I molecule is present at significantly different levels throughout the deer antler development and regeneration processes. Localization of IGF-I in chondrocytes and osteoblasts suggests that IGF-I may play an important role in cartilage and bone formation. In addition, it may have a variety of biophysical effects that influence the rapid growth of deer antlers.