To assess antioxidative protection by carnosic acid (CA) in combination with that of other low-molecular weight (M r ) antioxidants (␣-tocopherol [␣-T] and ascorbate [Asc]) in chloroplasts, we measured endogenous concentrations of these antioxidants, their redox states, and other indicators of oxidative stress in chloroplasts of three Labiatae species, differing in their CA contents, exposed to drought stress in the field. Damage to the photosynthetic apparatus was observed neither in CA-containing species (rosemary [Rosmarinus officinalis]) and sage [Salvia officinalis]) nor in CA-free species (lemon balm [Melissa officinalis]) at relative leaf water contents between 86% and 58%, as indicated by constant maximum efficiency of photosystem II photochemistry ratios and malondialdehyde levels in chloroplasts. The three species showed significant increases in ␣-T, a shift of the redox state of ␣-T toward its reduced state, and increased Asc levels in chloroplasts under stress. Lemon balm showed the highest increases in ␣-T and Asc in chloroplasts under stress, which might compensate for the lack of CA. Besides, whereas in rosemary and sage, the redox state of CA was shifted toward its oxidized state and the redox state of Asc was kept constant, lemon balm displayed a shift of the redox state of Asc toward its oxidized state under stress. In vitro experiments showed that both CA and Asc protect ␣-T and photosynthetic membranes against oxidative damage. These results are consistent with the contention that CA, in combination with other low-M r antioxidants, helps to prevent oxidative damage in chloroplasts of water-stressed plants, and they show functional interdependence among different low-M r antioxidants in chloroplasts.Mediterranean plants are exposed to a combination of environmental stress conditions, including low water availability, high irradiance, temperature fluctuations, and nutrient deprivation. Such stresses may lead to an imbalance between antioxidant defenses and the amount of activated oxygen species (AOS), resulting in oxidative stress (Smirnoff, 1993;Pastori and Foyer, 2002;Xiong et al., 2002). AOS are necessary for inter-and intracellular signaling (Doke, 1997;Foyer and Noctor, 1999), but at high concentrations they can cause damage at various levels of organization, including chloroplasts (Halliwell and Gutteridge, 1989;Asada, 1999). Apart from the xanthophyll cycle, photorespiration and other changes in metabolic activity, which may protect the chloroplasts from oxidative damage (Demmig-Adams and Adams, 1996;Kozaki and Takeba, 1996;Eskling et al., 1997;Osmond et al., 1997), a number of enzymatic and nonenzymatic antioxidants are present in chloroplasts that control oxygen toxicity (Smirnoff, 1993;Foyer et al., 1994;Asada, 1999). Carotenoids, ␣-tocopherol (␣-T, vitamin E), ascorbate (Asc, vitamin C), and glutathione help to maintain the integrity of the photosynthetic membranes under oxidative stress (Havaux, 1998; Noctor and Foyer, 1998;Asada, 1999;Smirnoff and Wheeler, 2000;Munné-Bosch and Ale...