Tomato plants were transferred to diflerent CO,^ mole fraction.s (3.S(), 700, lCSO and 1400//mol CO.^ mol ') 31 d after sowing (2 "" of full expansion) and the light saturated rate of photosynthesis (P",^^) of the unshaded .Sth leaf was measured at either an ambient CO^ mole fraction, C\ of 350/miol CO^ mol " [P^^^^ (350)] or at the mole fraction of CO,^ at which the plants were grown. At 60 "" and 95 "o leaf expansion, P^^^ of high CO^ grown plants measured at growth CO^, was greater than the P^^,, (350) of the ambient CO^ grown plants. However, by leaf maturity, P,,,,,, (growth CO^) declined linearly as growth CO^ concentration increased. P",^,^ (350) of plants exposed to elevated C().^ up to 60'\, leaf expansion had not acclimated to high CO.^. At 95 "" leaf expansion, f^,,,,. (350) was smaller in the high COj grown plants. Z^^^,, (350) was predicted from Rubisco in vitro carboxylation capacity using tomato Rubisco kinetic constants. By 95% leaf expansion, high CO.^ grown plants showed similarities to the response of plants to low nitrogen supply, in terms of Rubisco and chlorophyll content. The observed and theoretical relationships between the initial slopes of the P,,,^^/C^ responses and Rubisco activity were statistically equivalent. Both short-term and long-term effects of elevated CO^ on dark respiration (/?") were also investigated at two stages of leaf development (50 and 100 "o expansion). /?" (growth CO.^) was smaller for the high CO^ grown plants compared with the control plants, whereas /«" (350) was either equal or greater for the plants grown in high CO^.