In tomato {Lycopersicon esculentum L.) plants, net carbon dioxide exchange rate (P]si) response curves to both irradiance (/) and short-term [CO2] were similar for plants grown at both 350 and 700 cm3(C02) m'^. However, water vapor conductance (gHjo) of plants grown at high [CO2] was less sensitive to short term [CO2] variations, when measured at low vapor pressure difference, and was larger than the conductance of "ambient [CO2]" plants when both were exposed to high [CO2]. Pn and gHjO under high I increased with temperature over the range 18 to 32 °C. of plants grown in both [CO2] treatments increased at most about 25 % from 350 to 700 cm3 m-3 at 18 and 25 °C, and decreased when exposed to 1000 cm^ m'^ at these temperatures. Thus increasing atmospheric [CO2] might not increase P^ by as much as expected and water use of crops might not decrease.