The present study was designed to investigate how rat hyaline cartilages at various sites in vivo express the gene and protein of type I collagen using in situ hybridization and immunohistochemistry. The gene of pro alpha 1(I) collagen was expressed by chondrocytes in articular cartilage, and the protein of type I collagen was identified in the cartilage matrix. In contrast, growth plate cartilage expressed the gene of pro alpha 1(I) collagen, but no protein of type I collagen. Neither gene nor protein of type I collagen was expressed in cartilages of trachea and nasal septum. The present study suggested that expression of type I collagen in hyaline cartilages may be regulated tissue-specifically at gene and/or protein levels.