Salinity is among the environmental factors that affect plant growth and development and constrain agricultural productivity. Salinity stress triggers increases in cytosolic free Ca(2+) concentration (Ca(2+)i) via Ca(2+) influx across the plasma membrane. Salinity stress, as well as other stresses, induces the production of reactive oxygen species (ROS). It is well established that ROS also triggers increases in Ca(2+)i. However, the relationship and interaction between salinity stress-induced Ca(2+)i increases and ROS-induced Ca(2+)i increases remain poorly understood. Using an aequorin-based Ca(2+) imaging assay we have analyzed Ca(2+)i changes in response to NaCl and H2O2 treatments in Arabidopsis thaliana. We found that NaCl and H2O2 together induced larger increases in Ca(2+)i in Arabidopsis seedlings than either NaCl or H2O2 alone, suggesting an additive effect on Ca(2+)i increases. Following a pre-treatment with either NaCl or H2O2, the subsequent elevation of Ca(2+)i in response to a second treatment with either NaCl or H2O2 was significantly reduced. Furthermore, the NaCl pre-treatment suppressed the elevation of Ca(2+)i seen with a second NaCl treatment more than that seen with a second treatment of H2O2. A similar response was seen when the initial treatment was with H2O2; subsequent addition of H2O2 led to less of an increase in Ca(2+)i than did addition of NaCl. These results imply that NaCl-gated Ca(2+) channels and H2O2-gated Ca(2+) channels may differ, and also suggest that NaCl- and H2O2-evoked Ca(2+)i may reduce the potency of both NaCl and H2O2 in triggering Ca(2+)i increases, highlighting a feedback mechanism. Alternatively, NaCl and H2O2 may activate the same Ca(2+) permeable channel, which is expressed in different types of cells and/or activated via different signaling pathways.