A greenhouse experiment was performed in order to investigate the effects of different levels of water stress on leaf water potential (ψw), stomatal resistance (rs), protein content and chlorophyll (Chi) content of tomato plants (Lycopersicon esculentum Mill. cv. Nikita). Water stress was induced by adding polyethylene glycol (PEG 6 000) to the nutrient solution to reduce the osmotic potential (ψs). We investigated the behavior of anti-oxidant enzymes, such as catalase (CAT) and superoxide dismutase (SOD), during the development of water stress. Moderate and severe water stress (i.e.ψs= -0.51 and -1.22 MPa, respectively) caused a decrease in ψw for all treated (water-stressed) plants compared with control plants, with the reduction being more pronounced for severely stressed plants. In addition, rs was significantly affected by the induced water stress and a decrease in leaf soluble proteins and Chi content was observed. Whereas CAT activity remained constant, SOD activity was increased in water-stressed plants compared with unstressed plants. These results indicate the possible role of SOD as an anti-oxidant protector system for plants under water stress conditions. Moreover, it suggests the possibility of using this enzyme as an additional screening criterion for detecting water stress in plants.