Rates of net CO2 uptake were examined in developing leaves of Hydrocotyle bonariensis. Leaves that developed under high photosynthetically active radiation (48 mol m(-2) day(-1) PAR) were smaller, thicker, and reached maximum size sooner than did leaves that developed under low PAR (4.8 mol m(-2) day(-1)). Maximum net CO2 uptake rates were reached after 5 to 6 days expansion for both the low and the high PAR leaves. Leaves grown at high PAR had higher maximum photosynthetic rates and a higher PAR required for light saturation but showed a more rapid decline in rate with age than did low PAR leaves. To assess the basis for the difference observed in photosynthetic rates, CO2 diffusion conductances and the mesophyll surface available for CO2 absorption were examined for mature leaves. Stomatal conductance was the largest conductance in all treatments and did not vary appreciably with growth PAR. Mesophyll conductance progressively increased with growth PAR (up to 48 mol m(-2) day(-1)) as did the mesophyll surface area per unit leaf area, but the cellular conductance exhibited most of its increase at low PAR (up to 4.8 mol m(-2) day(-1)).