We have previously shown that N-cadherin expression is associated with tumor invasion, and that some cancer cells respond to specific extracellular matrix molecules by up-regulating N-cadherin. Pancreatic cancer is characterized by excessive deposition of type I collagen. Here, we show that human pancreatic cancer cells respond to collagen I, but not other matrices, by increasing motility and up-regulating mesenchymal markers, including N-cadherin. Both collagen I–mediated motility and metastasis in a mouse model for pancreatic cancer were inhibited by N-cadherin knockdown. Furthermore, inhibiting c-Jun NH₂-terminal kinase (JNK) with chemical inhibitors or short hairpin RNA abrogated all collagen I–induced changes. We show that JNK1 is activated in response to collagen I, which increases tumorigenesis by up-regulating N-cadherin expression and by increasing motility. (Cancer Res 2006; 66(24): 11745-53)