SUMMARYThe leaf diffusive resistance to the transfer of water vapour and CO2 exchange in rice plants (Oryza sativa L., ev. IR‐20) measured from the late vegetative stage to maturity, varied in response to differing CO2 enrichments. The leaf diffusive resistance of plants treated with 900 μ11−l CO2 ranged from one‐half to twice that of the control plants (330μl l−1 CO2).This difference was more pronounced during the heading and ripening stages. In plants treated with 600 μ1 −1 CO2, the diffusive resistances appeared to be smaller than in the control plants for most of the day. In the diurnal cycles, resistances were highest during the early morning and before sunset, the maximum again occurring in the plants treated with 900μ11 −1 CO2.The rate of transpiration was inversely related to the diffusive resistance, and was 20 to 100% lower in the plants treated with 900 μ1 1−1 CO2 than in the controls, both in the seasonal and diurnal cycles. In plants treated with 600 μ1 1−1 CO2, the rate of transpiration was lower than in the controls in the morning but distinctly higher during the afternoon. However, the integrated total daily transpiration in 600 μ1 1−1 CO2 did not exceed that of the controls. In all cases, CO2‐treated plants, through their increased biomass production and economic yield, appeared to have a higher water use efficiency than the control plants.