Seedlings of tomato (Lycopersicon esculentum Mill.) were grown in nutrient solution to investigate the changes in malondialdehyde (MDA) and proline content, and activities of antioxidative enzymes, plasmalemma H(+)-ATPase, and tonoplast H(+)-ATPase and H(+)-PPase to iso-osmotic stress of Ca(NO(3))(2) (80 mmol/L) and NaCl (120 mmol/L). The results indicated that both Ca(NO(3))(2) and NaCl stress significantly increased superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) activities in leaf cytosols and chloroplasts. The extent of increase caused by NaCl was higher than that by Ca(NO(3))(2). The activities of SOD, CAT and APX in mitochondria were increased under Ca(NO(3))(2) stress and decreased under NaCl stress. Salt stress enhanced the activities of plasmalemma H(+)-ATPase, tonoplast H(+)-ATPase and H(+)-PPase. The increases in tonoplast H(+)-ATPase and H(+)-PPase activity were much higher under Ca(NO(3))(2) stress than under NaCl stress. The MDA and proline content increased under salt stress, especially under NaCl stress. From the results, it can be concluded that both salt inhibited the plant growth, the effect of NaCl being much heavier.