Metabolism of glutamic acid, γ-aminobutyric acid, and proline in tobacco leaves during the day and night has been studied by using isotopic labeled compounds. Biosynthesis of proline from glutamate was very active in the day but slow at night; this indicates that light has an important role in the synthesis. γ-Aminobutyrate was very readily transformed into organic acids, and most of the activity was found in malate, succinate, and citrate. Accordingly, in tobacco leaves γ-aminobutyrate seems to be oxidized to succinate via succinic semialdehyde and to enter the tricarboxylic acid cycle. Oxidation of proline to glutamate took place very slowly in tobacco leaves both in the day and night without any appreciable difference. Reactions of proline in the stem and roots were similar to those in leaves. Accumulation of the activity of glutamate and γ-aminobutyrate in aspartate and citrate was higher at night than in the day.