Abstract: Dryland crops generally favor NO3--N,but some NH4+-N is usually beneficial to them.To determine the characteristics of nitrate uptake,two cultivars of lettuce(Lactuca sativa L.),Sx1 and Nrnct,were hydroponically grown in nutrient solution,i.e.100% NO3--N,10% NH4+-N + 90% NO3--N,10% NH4+-N + 100% NO3--N.The results indicated that NO3- uptake rate by lettuce increased with NO3- content,while the increase rate decreased.Compared with the treatment of 100% NO3--N,the rates of NO3- uptake by lettuce decreased significantly in the treatments of 10% NH4+-N + 90% NO3-N and 10% NH4+-N + 100% NO3--N.This indicated that NH4+-N in nutrient solution had significant effects on the rates of NO3- uptake.The Vmax of NO3- uptake kinetics in the treatment of 100% NO3--N was significantly higher than that of the treatments of 10% NH4+-N + 90% NO3--N and 10% NH4+-N + 100% NO3--N.The Km of NO3- uptake kinetics in the treatment of 100% NO3--N was lower than that of the treatments of 10% NH4+-N + 90% NO3--N and 10% NH4+-N + 100% NO3--N,but the difference was not significant.NH4+-N inhibited NO3- uptake,mainly due to the higher Vmax.Different cultivars differed in nitrate uptake characteristics.The NO3- uptake rate of Sx1 was higher than that of Nrnct in solution,and the difference increased with NO3- content.When there was NH4+-N in nutrient solution,Vmax of Nrnct decreased more than that of Sx1,and Km of Nrnct increased less than that of Sx1.