The mammary gland, during post-lactational involution, is subjected to extensive tissue reconstruction. This process is governed by the concerted expression of extracellular-matrix-degrading enzymes and their inhibitors. During carcinogenesis, the invasive growth of tumor cells is characterized by the penetration of the basement membrane and stromal invasion. We compared the expression of the tissue-remodeling enzymes stromelysin-1, a matrix metalloproteinase, and its inhibitor, tissue inhibitor of metalloproteinase-1 (TIMP-1), during mammary gland involution and carcinogenesis in mouse. In involuting mammary glands, stromelysin-1 was expressed in myoepithelial cells, whereas TIMP-1 was confined to the stromal tissue. To analyze the involvement of these tissue-remodeling genes in tumor development, we examined mammary tumors of transgenic mice expressing either the activated Ha-ras or c-myc oncogene under the control of a milk-protein gene promoter. In the undifferentiated and metastasizing Ha-ras-induced tumors, stromelysin-1 expression was comparable to that seen in involution, whereas TIMP-1 expression was greatly elevated. During Ha-ras-induced carcinogenesis, stromelysin-1 expression was first detected in the myo-epithelial cells surrounding preneoplastic lesions. In contrast, in the well-differentiated and non-metastatic mammary tumors induced by c-myc, no expression of either gene was observed. Thus, expression of stromelysin-1 and TIMP-1 is confined to the aggressively growing tumors and is induced in the earliest stages of carcinogenesis.