Exposing plants to long-term CO2 enrichment generally leads to increases in plant biomass, total leaf area and alterations on leaf net photosynthetic rates, stomatal conductance and water use efficiency. However, the magnitude of such effects is dependent on the availability of other potentially limiting resources. The aim of our study was to elucidate the effects of elevated CO2, applied at different temperature and water availability regimes, on nodulated alfalfa plants. Regardless of water supply, elevated CO2 enhanced plant growth, especially when combined with increased temperature although no differences were detected until 30 days of treatment. Absence of differences in leaf relative growth rate, and gas exchange measurements, suggested that plants grown in a low water regime adjusted their growth to the amount of available water. Elevated CO2 enhanced water use efficiency because of reduced water consumption and a greater dry mass production. Increased dry matter production of plants grown under elevated CO2 and temperature was the result of stimulated photosynthetic rates, greater leaf area and water use efficiency. Lack of CO2 effect on photosynthesis of plants grown at ambient temperature might be consequence of down-regulation phenomena. Plants grown at 700 μmol mol-1 CO2 maintained control nitrogen levels, discarding enhanced nitrogen availability as the main factor explaining enhanced dry matter. This work was supported by the Spanish Science and Technology Ministry (BFI2000-0154) and Fundación Universitaria de Navarra. I. Aranjuelo was the recipient of a research grant from Spanish Science and Technology Ministry (FP2000-52313). The temperature gradient tunnels were built with funds from the Spanish Commission of the Science and Technology (AMB96-0396). The technical co-operation of R. Martín, A. Urdiain and A. Verdejo is acknowledged. Professor Robert S. Nowak's (University of Nevada, Reno, USA) valuable help in reviewing this manuscript is greatly appreciated.