The sensitivity of stomatal conductance to changes of CO(2) concentration and leaf-air vapor pressure difference (VPD) was compared between two C(3) and two C(4) grass species. There was no evidence that stomata of the C(4) species were more sensitive to CO(2) than stomata of the C(3) species. The sensitivity of stomatal conductance to CO(2) change was linearly proportional to the magnitude of stomatal conductance, as determined by the VPD, the same slope fitting the data for all four species. Similarly, the sensitivity of stomatal conductance to VPD was linearly proportional to the magnitude of stomatal conductance. At small VPD, the ratio of intercellular to ambient CO(2) concentration, C(i)/C(a), was similar in all species (0.8-0.9) but declined with increasing VPD, so that, at large VPD, C(i)/C(a) was 0.7 and 0.5 (approximately) in C(3) and C(4) species, respectively. Transpiration efficiency (net CO(2) assimilation rate/transpiration rate) was larger in the C(4) species than in the C(3) species at current atmospheric CO(2) concentrations, but the relative increase due to high CO(2) was larger in the C(3) than in the C(4) species.