Field-grown olive trees (Olea europaea L. cv. Chemlali) were used over two growing seasons to determine the effects of different saline water irrigation levels on levels of proline and chlorophyll contents and activities of superoxide dismutase (SOD), polyphenol oxidase (PPO), ascorbate peroxidase (APX), and catalase (CAT). The plants were irrigated with fresh water (FW; ECe = 1.2 dS m(-1)) and saline water (SW; ECe = 7.5 dS m(-1)). Leaf water relations (relative water content, water potential), photosynthetic activity, and leaf chlorophyll content decreased under irrigation with saline water. In spring 2005, net photosynthesis of young leaves was 24.5 and 14.9 mu mol m(-2) s(-1) in FW- and SW-treated plants, respectively. In old leaves, these rates were 20.2 and 12.2 mu mol m(-2) s(-1), respectively. The relative reduction of net photosynthesis in SW-treated plants varied from 39 to 46% and from 39 to 61%, compared to FW-treated plants during the first and second crop seasons, respectively. The relative reduction of leaf chlorophyll (a + b) content under high water salinity level exceeds 50%, compared to FW-treated plants. However, proline content and activities of SOD, CAT, and APX increased under saline water irrigation. The increase of proline content was more important in leaves than in roots. In young leaves, the increment of antioxidant activities in SW-treated plants was 2.67, 3.61, and 1.85 times, respectively, for SOD, APX, and CAT, compared to FW-treated plants. From these results, interaction between antioxidant defense system and proline contents seems to be involved in the salt tolerance mechanisms of Chemlali olive tree.