Regulation of long bone growth by growth hormone and other endocrine factors is mediated by the local synthesis of IGF-I in the growth plate. Recent evidence suggests that different regions of the growth plate exhibit variable growth rates. To investigate whether IGF-I gene expression in the growth plate differs in relation to growth, we examined the distribution of IGF-I mRNA and peptide using in situ hybridization and immunohistochemistry, respectively, in the tibiae of 18-week-old rats (n = 6). Osteoblasts were identified by osteocalcin immunoreactivity, and osteoclasts by tartrate-resistant acid phosphatase (TRAP) histochemistry. The abundance of IGF-I mRNA in growth plate chondrocytes was quantified by counting the autoradiographic signal associated with each cell. IGF-I mRNA was identified in chondrocytes of both the proliferative and hypertrophic zones of the growth plate. Cells in the marginal regions of both zones contained significantly more IGF-I mRNA than those in the central region (p < 0.05). In addition, IGF-I mRNA levels were greater in the periphery of the growth plate on the medial side of the tibia (p < 0.05) in which there was more active growth than the lateral side. IGF-I immunoreactivity was present predominantly in the hypertrophic zone chondrocytes and no regional differences in its distribution were observed. IGF-I mRNA and peptide were also identified in periosteal fibroblasts, notably at sites of muscle attachment to bone, and in osteoblasts at active sites of bone remodelling in the periosteal, endocortical, and endosteal bone envelopes. In the TRAP-positive osteoclasts, IGF-I immunoreactivity, but not IGF-I mRNA, was detected. In addition, both IGF-I mRNA and peptide were identified in the hemopoietic cells of the metaphyseal bone marrow, whereas only IGF-I immunoreactivity was detectable in the diaphysis. We conclude that, in the tibiae of mature rats: (i) IGF-I gene expression in the growth plate is related to its growth and/or synthetic activity; and (ii) the presence of IGF-I in osteoblasts and osteoclasts suggests its involvement in active bone growth and remodeling.