The signaling pathways leading to cellular protection or cell death following exposures to heavy metals have not been fully clarified.  c-Jun NH₂-terminal kinase (JNK), also known as stress-activated protein kinase (SAPK), is a member of mitogen-activated protein kinase (MAPK) family.  JNK is activated primarily by cellular stresses and inflammatory cytokines, and has been known to be involved in apoptosis, inflammation or differentiation. For the activation, JNK requires the dual phosphorylation of Thr and Tyr residues located in a Thr-Pro-Tyr motif between kinase subdomains VII and VIII.  This phosphorylation is catalyzed by the dual specific kinases MKK4 and MKK7.  Treatment with cadmium chloride (CdCl₂) activates JNK, and induces the expression of c-fos, c-jun and other stress response genes.  In order to reveal the toxicological significance of cadmium-induced JNK activation, blocking of JNK pathway by two distinct ways has been applied. (1) Treatment with a macrocyclic nonaketide LL-Z1640-2 suppressed the phosphorylation and activation of JNK in NIH3T3 cells exposed to CdCl₂. (2) In mkk4^-/- and mkk7^-/- mouse embryonic stem (ES) cells, CdCl₂-induced phosphorylation and activation of JNK were suppressed significantly.  LL-Z1640-2 and mouse ES cells lacking upstream JNK activator seem to be useful to analyze functions and signaling pathways of JNK activation induced by environmental stresses including toxic metals.