Stomatal conductances, CO₂ assimilation, transpiration and intercellular CO₂ mol fractions of Eucalyptus grandis leaves were measured in the field using a portable, controlled environment cuvette. Test leaves were subjected to a range of temperatures, humidities, photon irradiances and external CO₂ mol fractions. An empiral function, g_sw = g₀ + g₁ Ah_s/(c_s-I'), was able to account for steady- state stomatal conductances g*sw, over a wide range of environmental conditions and leaf photosynthetic capacities. In this equation, termed the stomatal constraint function, A is CO₂ assimilation rate, h_s and c_s are relative humidity and CO₂ mol fraction at the leaf surface respectively, I' is the CO₂ compensation point, g₀ is conductance at A = 0 and g_l is an empirical coefficient. Equations describing the supply of CO₂ through stomata and demand for CO₂ in photosynthesis were solved simultaneously with the constraint function to give a combined model of stomatal conductance, CO₂ assimilation and intercellular CO₂ mol fraction in terms of external environmental factors and several parameters describing C₃ photosynthesis. The model provided a good description of experimental observations.