The c-Jun NH₂-terminal kinase (JNK) has been implicated in the function of transforming growth factor β (TGF-β). To test the role of JNK, we examined the effect of compound disruption of the murine genes that encode the ubiquitously expressed isoforms of JNK (Jnk1 and Jnk2). We report that JNK-deficient fibroblasts isolated from Jnk1^−/− Jnk2^−/− mice constitutively express TGF-β1. Complementation studies demonstrate that JNK is a repressor of Tgf-β1 gene expression. This mechanism of regulation of TGF-β1 expression by JNK represents an unexpected form of cross-talk between two important signaling pathways. Together, these data demonstrate that the JNK pathway may contribute to the regulation of autocrine TGF-β1-mediated biological responses in vivo.