The tissue inhibitor of metalloproteinases-1 (TIMP-1) has at least 2 independent functions, i.e., regulation of matrix metalloproteinases and erythroid-potentiating activity. We investigated the effects of TIMP-1 over-expression on tumor growth, using cloned lines derived from a TIMP-1-transfected rat breast carcinoma cell line. The in vitro growth rate of the TIMP-1-transfected clones was indistinguishable from that of the control. In contrast, the highest TIMP-1-producing clone (159.0 ng/ml), designated as T-H, formed 4.6-fold larger s.c. tumors than did the control after 14 days. Tumors derived from an intermediate TIMP-1-producing clone (45.4 ng/ml), designated as T-M, were 1.9-fold larger than the control. TIMP-1 over-expression was associated with increased vascular endothelial growth factor (VEGF) expression, vascularization and proliferative activity of the s.c. tumors. Similar to the rat breast carcinoma cells, transfection of TIMP-1 cDNA into the human breast carcinoma cell line MCF-7 resulted in up-regulation of VEGF, with a linear relationship between TIMP-1 and VEGF production in 9 cell clones examined. There was, however, no change in VEGF expression when the rat and human breast carcinoma cell lines were exposed to exogenous recombinant TIMP-1. Our findings suggest that over-expression of TIMP-1 confers growth advantage on breast carcinoma cells in vivo and that up-regulation of VEGF expression may play an important role in this TIMP-1mediated, growth-stimulating effect.