Stomatal response to changes in leaf to air vapor pressure difference (VPD) and its influence on total gas exchange were measured for leaves of Podocarpus oleifolius and P. rospigliosii in an open gas exchange system. Stomatal conductance (g) of both species declined as VPD increased. Once a critical VPD was attained, the decrease in g was sufficient to decrease transpiration rate in spite of increasing VPD. This response pattern suggested feedforward control of stomatal response to humidity rather than negative feedback control based on changes in leaf water status. Coupling between CO2 assimilation and g and inherent water-use efficiency were greater in P. oleifolius. Stomatal response to humidity in P. oleifolius was consistent with a recent hypothesis that stomata optimize CO2 assimilation with respect to a given level of water loss by maintaining constant the ratio of the sensitivities of transpiration rate (E) and assimilation rate (A) to changes in g (.delta.E/.delta.g/.delta.A/.delta.g). This gain ratio did not remain constant in P. rospigliosii as VPD was varied. The possible ecological basis for these differences in gas exchange behavior is discussed.