To investigate the effect of nitrogen forms on the growth and physiological characteristics of the Lactuca sativa L.seedlings,we treated overground parts and roots of L.sativa L.seedlings with different mixing ratios of ammonium nitrogen and nitrate nitrogen using hydroponic technique.Our results showed that:(1) with the increasing of the ratio of ammonium nitrogen in nutrient solution,the nitrate content of overground parts and roots of L.sativa L.showed a reducing trend and was up to the standard of green vegetables.(2) With the increasing of the ratio of ammonium nitrogen in nutrient solution,the overground parts showed reduced content of organic acid in the early stage but a great increase in later stage.Soluble carbohydrate content in overground parts increased when NH₄⁺increased.Soluble protein content,however,declined first and increased later.When NO₃^-∶NH₄⁺ was 5∶5,the content of organic acid in overground parts was minimum while soluble protein reached the maximum.On the contrary,organic acid content in roots exhibited the opposite trend,with an elevation in early stage and reduction in later stage.The maximum was detected when NO₃^-∶NH₄⁺ was 5∶5.Soluble carbohydrate had the highest level when NO₃^-∶NH₄⁺was 7.5∶2.5 in roots.Soluble protein content in roots had maximum level with total NO₃^--N in nutrient solution and declined when NH₄⁺increased.(3) The SOD activity of overground parts and roots of L.sativa L.increased first and declined later when NH₄⁺increased.The SOD activity in overground parts was the highest when NO₃^-∶NH₄⁺ was 5∶5 ，but it reached the maximum when NO₃^-∶NH₄⁺was 7.5∶2.5 in roots.On the contrary,the contents of MDA and proline as well as the activities of POD and CAT exhibited the opposite trend both in overground parts and in roots.(4) With the increasing of the ratio of ammonium nitrogen in nutrient solution,rootshoot ratio declined while the dry weight of both overground parts and roots increased first and declined later.When NO₃^-∶NH₄⁺ was 7.5∶2.5,the dry weight of both overground parts and roots were the highest,and rootshoot ratio was optimal.In summary,different forms of nitrogen had remarkable effect on the growth and physiological characteristics of both overground parts and roots of L.sativa L.seedlings，and roots physiological response was more sensitive.When NO₃^-∶NH₄⁺ was 7.5∶2.5 in nutrient solution,the coordinated growth of the overground parts and roots was optimal,and the degree of stress was minimum.