Mitogen-activated protein kinase (MAPK) is activated by various biotic and abiotic stresses. Salt stress induces two well-characterized MAPK activating signaling molecules, phosphatidic acid (PA) via phospholipase D and phospholipase C, and reactive oxygen species (ROS) via nicotinamide adenine dinucleotide phosphate (NADPH)-oxidase. In our previous study, the activity of soybean MAPK, GMK1 was strongly induced within 5 min of 300 mM NaCl treatment and this early activity was regulated by PA. In this study, we focused on the regulation of GMK1 at the later stage of the salt stress, because its activity was strongly persistent for up to 30 min. H(2)O(2) activated GMK1 even in the presence of PA generation inhibitors, but GMK1 activity was greatly decreased in the presence of diphenyleneiodonium, an inhibitor of NADPH-oxidase after 5 min of the treatment. On the contrary, the n-butanol and neomycin reduced GMK1 activity within 5 min of the treatment. Thus, GMK1 activity may be sustained by H(2)O(2) 10 min after the treatment. Further, GMK1 was translocated into the nucleus 60 min after NaCl treatment. In the relationship between GMK1 and ROS generation, ROS generation was reduced by SB202190, a MAPK inhibitor, but was increased in protoplast overexpressing TESD-GMKK1. However, these effects were occurred at prolonged time of NaCl treatment. These data suggest that GMK1 indirectly regulates ROS generation. Taken together, we propose that soybean GMK1 is dually regulated by PA and H(2)O(2) at a time dependant manner and translocated to the nucleus by the salt stress signal.