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The photosynthetic response of Larrea tridentata Cav,, an evergreen Mojave Desert shrub, to elevated atmospheric CO2 and drought was examined to assist in the understanding of how plants from water-limited ecosystems will respond to rising CO2, We hypothesized that photosynthetic down-regulation would disappear during periods of water Limitation, and would, therefore, likely be a seasonally transient event. To test this we measured photosynthetic, water relations and fluorescence responses during periods of increased and decreased mater availability in two different treatment implementations: (1) from seedlings exposed to 360, 550, and 700 mu mol mol(-1) CO2 in a glasshouse; and (2) from intact adults exposed to 360 and 550 mu mol mol(-1) CO2 at the Nevada Desert FACE (Free Air CO2 Enrichment) Facility. FACE and glasshouse wed-watered Larrea significantly down-regulated photosynthesis at elevated CO2, reducing maximum photosynthetic rate (A(max)), carboxylation efficiency (CE), and Rubisco catalytic sites, whereas droughted Larrea showed a differing response depending on treatment technique. A(max) and CE were lower in droughted Larrea compared with well-watered plants, and CO2 had no effect on these reduced photosynthetic parameters. However, Rubisco catalytic sites decreased in droughted Larrea at elevated CO2. Operating C-i increased at elevated CO2 in droughted plants, resulting in greater photosynthetic rates at elevated CO2 as compared with ambient CO2. In well-watered plants, the changes in operating C-i, CE andA(max) resulted in similar photosynthetic rates across CO2 treatments. Our results suggest that drought can diminish photosynthetic down-regulation to elevated CO2 in Larrea, resulting in seasonally transient patterns of enhanced carbon gain. These results suggest that water status may ultimately control the photosynthetic response of desert systems to rising CO2.