Experiments were carried out to determine the role of Raf-1 kinase in the development of drug resistance and apoptosis induced by paclitaxel. In the present study, paclitaxel sensitivity, Raf-1 activity and mitogen-activated protein kinases activation were compared in two cell lines: parental human breast cancer cells and its drug resistant variant (MCF-7/Adr) cells. Paclitaxel treatment of parental MCF-7 cells caused a marked inhibition of Raf-1 kinase activity, concomitant with its mobility shift after 18 h exposure. In addition, paclitaxel greatly increased c-Jun N-terminal protein kinase (JNK) activity whereas showing a small enhancing effect on extracellular-regulated kinases (ERK) activity. Interestingly, MCF-7/Adr cells have lower basal Raf-1 activity, yet have much higher basal ERK activity than parental cells. However, it appeared that PD 98059, which turns off ERK through mitogen-activated protein kinase kinase (MEK) inhibition, enhanced basal Raf-1 kinase activity in MCF-7/Adr cells. Thus, the findings suggest that paclitaxel-induced apoptosis is mediated by JNK and occurs in parallel with suppression of the Raf-1 kinase activity in parental MCF-7 cells. In addition, down-regulation of Raf-1 kinase, which can be induced through the sustained ERK activation, may contribute to the development of acquired resistance in MCF-7/Adr cells.