Abstract: The content of the soluble protein and the activities of glutamine synthetase and glutamate dehydrogenase were determined in the development of cucumber(Cucumis sativus L.) cotyledon at the presence or absence of nitrate. In the early days of the development, the level of the protein and the activities of GS, NADH-GDH, and (NAD⁺-GDH) in each pair of cotyledons increased whether exogenous nitrogen was existent or not. After 4 days, the level of the protein was constant roughly, though it was declined little at the presence of the exogenous nitrogen. The activities of GS and NADH-GDH fell gradually, but NAD⁺-GDH was raised markedly. At the absence of the exogenous nitrogen, the levels of the protein, GS, NADH-GDH, and (NAD⁺-GDH) decreased gradually at the same stage. In the whole course of the development, the activities of GS and NAD⁺-GDH increased by exogenous nitrogen, and NAD⁺-GDH activity was promoted remarkably in the late stage of the development particularly.