Summer diurnal variations of photosynthesis and α-tocopherol content were measured in relation to natural drought in field-grown rosemary (Rosmarinus officinalis L.) and lemon balm (Melissa officinalis L.) plants. During the summer relative water contents (RWC) of ca. 40% in Rosmarinus officinalis and ca. 30% in Melissa officinalis were attained, indicating severe drought. Both species showed similar diurnal patterns of net CO2 assimilation rates (A) with a wide plateau of maximum photosynthesis at midday in the absence of drought and one peak of maximum photosynthesis early in the morning under drought conditions. Net CO2 assimilation rates decreased by ca. 75% due to drought in both species. Melissa officinalis plants showed a significant decrease in the relative quantum efficiency of PSII photochemistry (Φ(PSII)), ratio of variable to maximum fluorescence yield (F(v)/F(m)) and chlorophyll content of leaves by ca. 25% under drought conditions at midday. In contrast, Φ(PSII), F(v)/F(m) and chlorophyll content remained constant throughout the experiment in R. officinalis plants. Although the non-photochemical quenching of chlorophyll fluorescence increased from ca. 1.8 to 3 and the α-tocopherol content rose fifteen fold in both species in response to drought, only R. officinalis plants were able to avoid oxidative damage under drought conditions by the joint increase of carotenoids and α-tocopherol.