Observations of nonuniform photosynthesis across leaves cast doubt on internal CO(2) partial pressures (p(i)) calculated on the assumption of uniformity and can lead to incorrect conclusions about the stomatal control of photosynthesis. The problem can be avoided by measuring p(i) directly because the assumptions of uniformity are not necessary. We therefore developed a method that allowed p(i) to be measured continuously in situ for days at a time under growth conditions and used it to investigate intact leaves of sunflower (Helianthus annuus L.), soybean (Glycine max L. Merr.), and bush bean (Phaseolus vulgaris L.) subjected to high or low leaf water potentials (psi(w)) or high concentrations of abscisic acid (ABA). The leaves maintained a relatively constant differential (Deltap) between ambient CO(2) and measured p(i) throughout the light period when water was supplied. When water was withheld, psi(w) decreased and the stomata began to close, but measured p(i) increased until the leaf reached a psi(w) of -1.76 (bush bean), -2.12 (sunflower) or -3.10 (soybean) megapascals, at which point Deltap = 0. The increasing p(i) indicated that stomata did not inhibit CO(2) uptake and a Deltap of zero indicated that CO(2) uptake became zero despite the high availability of CO(2) inside the leaf. In contrast, when sunflower leaves at high psi(w) were treated with ABA, p(i) did not increase and instead decreased rapidly and steadily for up to 8 hours even as psi(w) increased, as expected if ABA treatment primarily affected stomatal conductance. The accumulating CO(2) at low psi(w) and contrasting response to ABA indicates that photosynthetic biochemistry limited photosynthesis at low psi(w) but not at high ABA.