In order to investigate the effect of NH_4-N on respiration of tomato plants, the following two experiments were carried out. a) Detached leaves or roots were treated with a high concentration of NH_4-N solution for several hours. b) Plants were cultured in solutions containing comparatively low levels of NH_4-N (50-100 ppm) for a long period. When detached roots were treated with a high level of NH_4-N, oxygen uptake was more decreased than that in the check. When plants were cultured in a solution containing a comparatively low level of NH_4-N, respiration was accelerated in both leaves and roots. RQ (CO_2/O_2) values were lower in the leaves of NH_4-N fed plants than in those of NO_3-N fed ones. In roots, no difference in RQ values was found between the plants fed with these two nitrogen sources. The difference in RQ value in leaves was attributed to the fact that the oxygen uptake was greater than the carbon dioxide output in NH_4-N fed plants. Consequently, there appears to be a qualitative difference between respiration by roots and leaves. The degress of inhibition of O_2 uptake by malonate and azide were higher in leaves in NH_4-N fed plants than in NO_3-N fed ones, though no difference was found in inhibition by hydroxyquinoline. In roots, on the other hand, the degree of inhibition by hydroxyquinoline was remaekably higher in NH_4-N fed plants than in NO_3-N fed ones. These facts suggest that in the leaves iron-containing enzymes play an important part as the terminal oxidase in NH_4-N fed plants than in NO-N fed ones, while in the roots copper-containing enzymes have an important role in NH_4-N fed plants.