Although the synthesis of angiogenic factors in hypoxic regions of solid tumors is recognized as one of the critical steps in tumor growth and metastasis, the signal transduction pathway involved in hypoxic induction of basic fibroblast growth factor (bFGF) gene expression is still obscure. In the study described here, we investigated the intracellular responses to hypoxia and the mechanisms triggering the initiation of angiogenic activity in drug-resistant human breast carcinoma MCF-7/ADR cells. Northern blots showed an increase in the level of c-jun, c-fos, and bFGF mRNA during hypoxia. Gel mobility-shift analysis of nuclear extracts from hypoxia-exposed cells showed an increase in AP-1 binding activity. In addition, hypoxic treatment strongly activated c-Jun N-terminal kinase 1 (JNK1), leading to phosphorylation and activation of c-Jun. Expression of a dominant negative mutant of JNK1 suppressed hypoxia-induced JNK1 activation as well as bFGF gene expression. Taken together, hypoxia-induced bFGF gene expression is mediated through the stress-activated protein kinase (SAPK) signal transduction pathway.