OBJECTlVE To explore the mechanisms of tau hyperphosphorylation induced by calyculin A ( CA) in neuroblastoma cells and the effect of melatonin. METHODS N2a cells were treated with CA 5 nmol·L-1 , or CA with melatonin 50 μmol·L-1 , or CA with vitamin E ( Vit E ) 50 μmol·L-1 for 12 h. The level of tau phosphorylation at Ser422 ( recognized by R145d antibody) site and the level of phosphorylated c-Jun N-terminal kinases ( p-JNK ) and phosphorylated mitogen-activated protein kinase kinase 4 ( p-MKK4 ) were detected with immunoblotting, the level of malondialdehyde ( MDA ) was assayed with fluorimetry, and the activity of p38-mitogen activated protein kinase ( p38MAPK ) was assayed by radioimmunobloting. RESULTS CA treatment increased the level of phosphorylated tau at Ser422 site (1.70±0.19, 1.0, P<0.01), and melatonin attenuated the effect of CA (0.98±0.12, 1.70± 0.19, P<0.01). ln addition, CA treatment increased the level of MDA (μmol·g-1 protein:0.241±0.006, 0.141±0.006, P<0.01) and melatonin antagonized the increase of MDA induced by CA (μmol·g-1 protein:0.172±0.004, 0.193±0.005, 0.241±0.006, P<0.01) . CA treatment increased the level of p-JNK (1.91±0.27, 1, P<0.01) and p-MKK4 (1.81±0.09, 1, P<0.01) and melatonin antagonized the effect of CA induced increase of p-JNK (1.11±0.15, 1.91±0.27, P<0.01) and p-MKK4 (1.14±0.06, 1.81±0.09, P<0.01) without changing the level or activity of p38MAPK. Both JNK inhibitor ( SP600125 ) and MKK4/JNK transduction pathway inhibitor antagonized CA induced tau phosphorylation at Ser422 site and JNK phosphorylation. CONCLUSlON lnhibiton of JNK phosphorylation is possibly involved in the protection of melatonin on CA-induced tau hyperphosphorylation at Ser422 site.