Rumex obtusifolius plants were grown for several months in daylit environment chambers (Solardomes) force-ventilated with air containing 350 or 600 µ;mol mol–1 C02. Elevated CO2 was found to accelerate the natural ontogenic decline in photosynthesis, but did not reduce leaf duration. In both CO2 treatments photosynthetic rates declined progressively with increasing leaf age, the decline being greater for plants grown in elevated C02 such that rates became lower than in ambient CO2. The degree of CO2-induced photosynthetic down-regulation as determined by A/C1 analysis was found to be dependent on leaf age. The major contribution to the decline in photosynthesis was likely to be a reduction in Rubisco activity as changes in stomataland mesophyll limitations were small. Instantaneous water use efficiency (WUE1) was greater for plants in elevated CO2, but these values declined rapidly with leaf age, whereas in ambient CO2 values were always lower, but were maintained for longer. Growth analysis indicated an increased root:-shoot ratio for plants grown in elevated CO2, this occurring almost entirely as a result of increased root growth. Greater root proliferation and increased WUE1, are characteristics which should give this persistent and troublesome weed an increased competitive advantage under projected conditions of climate change