Abstract: The possible involvement organic metabolites and the antioxidative defence system in water drought tolerance were investigated in three olive cultivars. About 2 years old olive trees, grown in pots in greenhouse were subjected to increasing Water Deficit (WD) regime (5, 10, 20 and 40 days). After 40 days without irrigation, the biomass production, osmotic potential (Ψ_s) and turgor Potential (P) were decreased in the three olive cultivars and the lower reduction was observed in Zalmati. Under WD conditions, Chemlali and Zalmati showed a greater accumulation of proline associated with the better maintenance of plant water status and biomass production. Different cultivars developed certain antioxidative defense mechanisms including the accumulation of phenolic compounds and increased antioxidative enzymes activities. However, differences were observed between the three cultivars for these enzymatic and non-enzymatic antioxidants. Zalmati leaves accumulated the larger quantities of phenolic compounds followed by Chemlali which can improve their antioxidant responses. Superoxide Dismutase (SOD) activity was more enhanced in Zalmati than in Chemlali under water stress but was decreased in Chetoui. Catalase (CAT) activity was more enhanced in Chetoui under water stress. The activity of Peroxidase (POD) and Ascorbate Peroxidase (APX) increased under water stress and the higher levels were reached in Zalmati followed by Chemlali. In contrast, Polyphenol Oxidases (PPO) activity decreased under WD conditions in Chemlali and Zalmati, however this reduction was slightly in Chetoui. The lower increase in malondialdehyde content and H₂O₂ generation suggest that Zalmati was more able to maintain leaf cell integrity than Chemlali, especially Chetoui under severe water stress because of the more efficient osmoprotection and antioxidant defense systems. Thus, the great accumulation of phenolic compounds, the increased activity of SOD and the higher POD, especially APX activity might be linked to the better performance of the antioxidant defense in Zalmati olive cultivar under severe water deficit.