Abstract: Mongolian pine(Pinus sylvestris L.var.mongolica Litv.) is one of the most drought resistant tree species.It has been widely introduced on sandy soils in the northern region of China(north,northwest and northeast China) but a decline in forest growth and timber quality has occurred in the earliest Mongolian pine plantations on Keerqin sandy soils located at Zhanggutai,Liaoning Province.In order to study this decline problem,we carried out an experiment to examine the effects of the following two drought stresses:soil water(SW) stress and polyethylene glycol(PEG) stress,on photosynthesis of 2-year old Mongolian pine seedlings on sandy soils.As well,we tested the response of photosynthesis on different types of drought stresses. The results indicated no significant differences between the effects of SW and PEG stress,processed in 1 hr,on indices of instantaneous photosynthetic rates, stomatal conductance,transpiration rates and CO₂ concentration inside cells.When the soil water content was 40% of field water capacity,the seedlings started to become stressed and became heavily stressed at 20%.The cause of the stress was the change in stomatal conductance.The effects of PEG stress on photosynthetic indices showed the following characteristics:1) a 10% PEG stress had the same effect on treated seedlings as on seedlings of the control group and there was less stress on the 2-year old seedlings;2) 20% and 30% PEG stresses had the same effect and the values of instantaneous photosynthetic rates,stomatal conductance,transpirati on rates and CO₂ concentrations inside cells were very low and stable after a 2 hr stress period.At 20% field water capacity there was a slight SW stress effect on water use efficiency(WUE) of the seedlings;at 40% capacity,SW stress improved WUE.Under conditions of PEG stress,the WUE of stressed seedlings was lower than that of control seedlings within 4 hrs since the start of the stress.For the 20% and 30% PEG stresses,WUE was higher for treated seedlings than for the control after a 4 hr stress.These results show that Mongolian pine seedlings can endure and adapt to drought stresses under conditions of lower soil water content.Comparisons between PEG stresses at 10%,20% and 30% within 2 hrs and SW stresses at 40%,30% and 20% of field water capacity showed that the relative values of photosynthetic indices were very similar.Therefore,PEG stresses within 2 hrs had the same effect as SW stress within 7~8 days.