Transpiration per unit leaf area of Acacia farnesiana (L.) Willd. plants grown at a CO2 concentration ([CO2]) of 385 micromol x mol(-1) was about twice that of plants grown at 980 micromol x mol(-1). However, whes plants grown for more than a year at 980 micromol x mol(-1) were exposed to 380 micromol x mol(-1) for 9 days, they transpired at half the rate of those that had been grown at 380 micromol x mol(-1)1. Similarly, plants grown at 380 micromol x mol(-1), when exposed to 980 micromol x mol(-1), transpired at twice the rate of those grown at 980 micromol x mol(-1). Thus, the effects of elevated [CO2] on whole-plant transpiration, like those on photosynthesis, respiration and stomatal conductance, cannot reliably be extrapolated from measurements made during short-term exposure to elevated [CO2].