Net photosynthesis, transpiration and stomatal conductance of two-year-old Pinus taeda L. seedlings were compared under various environmental conditions. Responses to air temperature, irradiance, ambient CO(2) concentration, absolute humidity deficit and xylem pressure potential were examined. The seedlings exhibited little response to a wide range of absolute humidity deficits (7 to 16 g m(-3)) and temperatures (20 to 35 degrees C), but were sensitive to changes in water deficit, irradiance and CO(2) concentration. Net photosynthesis and stomatal conductance were linearly related under all of the environmental conditions measured. However, the gas phase limitation to photosynthesis was generally small (20 to 30%). It was concluded that although the stomatal response was closely coupled to changes in photosynthesis, internal limitations, rather than the rate of gaseous diffusion of CO(2), were primarily responsible for limiting photosynthesis.