In order to understand the mechanism(s) of pre-conditioning (PC) by increased levels of salinity that enhance transplant establishment in the field, we studied the possible roles of ethylene and oxidative stress in transplants of tomato (Solanum lycopersicum L.) during PC by saline treatments. Prior to transplanting, transplants were fed hydroponically for 6 d with a normal nutrient solution, or solutions containing various concentrations of NaCl, ethephon, or silver thiosulphate (STS), or various combinations of these chemicals. The transplants were then transferred to normal nutrient solution for 10 d to recover. Addition of up to 75 mM NaCl to the nutrient solution decreased root elongation slightly and significantly increased lateral root (LR) formation during PC, but enhanced the Subsequent recovery of roots and shoots. All the salinity treatments inhibited root ethylene production. However, the application of 20 mg l(-1) ethephon to the roots during PC increased ethylene concentrations, inhibited root elongation, increased LR formation, and stimulated subsequent root growth. An enhancement of root growth during recovery was also observed when 75 mM NaCl Plus 20 mg l(-1) ethephon was applied to the root system during PC. The ethylene blocker, STS, enhanced root elongation during PC, and acted synergistically with NaCl in enhancing subsequent root growth. As PC treatments, NaCl or ethephon both increased root malonyldialdehyde (MDA) and H2O2 concentrations, and increased root peroxidase and superoxide dismutase (SOD) activities: but. if NaCl or ethephon were combined with STS, the extent of these increases was diminished. Application of <= 1mM H2O2 at PC increased subsequent root and shoot growth in seedlings, but higher H2O2 concentrations were inhibitory. These results suggest that subsequent root growth upon recovery from transplantation benefits from PC applications of NaCl or ethephon and this might be due the enhancement of LR initiation during PC by these treatments, possibly through the signalling role of H2O2. STS alleviates oxidative stress in the root by inhibiting the action of ethylene and decreasing ethylene sensitivity, thereby promoting root recovery.These morphological and biochemical changes may explain the beneficial effect of PC on transplant development under field conditions.