Net photosynthesis of Pinus attenuata and P. coulteri seedlings was determined by CO2 analysis under limiting conditions of soil moisture and atmospheric CO2. As the CO2 limiting factor was removed by enhancement of atmospheric CO2, net phostosynthesis increased even when soil water potential was below.sbd.15 bars. This indicates that net photosynthesis of vegetation in dry climates will respond to increasing atmospheric CO2 in future years.