Interleukin 1 (IL-1) and tumor necrosis factor alpha (TNFalpha) are known to induce production of reactive oxygen species (ROS), which have been suggested to act as second messengers. Here we demonstrate that ROS production by bovine chondrocytes upon cytokine stimulation induces c-jun expression. Since c-jun expression is regulated by its own gene product via phosphorylation by c-Jun NH2-terminal kinases (JNKs), we investigated if cytokines and ROS could modulate JNK activity in chondrocyte monolayer cultures. Treatment of bovine chondrocytes with both IL-1 and TNFalpha leads to rapid induction of JNK activity, stimulating JNK activity 7- and 20-fold, respectively. Importantly, the observation that antioxidant treatment antagonizes IL-1 and TNFalpha activation of JNK provides strong evidence that ROS can act as mediators of JNK activity. Moreover, potent activation of JNK is also observed by direct addition of the ROS hydrogen peroxide (H2O2) to the chondrocyte cultures. Nitric oxide (NO), a multifunctional ROS, also appears to simulate JNK, albeit to a lesser extent. These findings identify JNK as another molecular target for the actions of NO and H2O2. In addition, the inhibitory effect of diphenyleneiodonium on JNK activation implicates the involvement of flavonoid-containing enzymes in the ROS-mediated signaling process. Overstimulation of JNK activity by excessive production of ROS may, therefore, underlie pathological conditions such as arthritis and cancer.