Plants of Rumex obtusifolius L. were grown in Solardomes under ambient and elevated (+ 250 μmol mol−1) mole fractions of CO2 and were exposed to two levels of herbivory by Gastrophysa viridula Degeer larvae. The herbivory treatment lasted 1 month, thereafter half of the plants were harvested and over the following month during a period of regrowth physiological measurements were made on the remaining plants. At the termination of the herbivory treatment uninfested plants showed no damage, whereas the low and high herbivore treatments caused 20–40% and 50–70% loss of leaf area as a proportion of total leaf area, respectively. The CO2 treatment did not affect the degree of defoliation. Total leaf area was not significantly affected by either CO2 or herbivory. Uninfested plants grown in elevated concentrations of CO2 showed increased growth, root-to-shoot ratios (RS), rates of photosynthesis and reduced stomatal conductance compared with uninfested plants grown in ambient CO2. A/C1 analysis revealed that plants grown in elevated CO2 showed reductions in Vcmax. For plants grown in ambient CO2 the high herbivory treatment led to increased rates of photosynthesis and decreased rates of dark respiration per unit leaf area, and caused increases in stomatal conductance and RS. For plants grown in elevated CO2 the high herbivory treatment increased plant biomass and RS. The increases in RS in response to elevated CO2 and herbivory appeared to be additive. Defoliation did not reduce the degree of photosynthetic down-regulation caused by growth in elevated concentrations of CO2, but appeared to reduce the rate of ontogenic decline in photosynthesis in ambient CO2.