The influence of the nitrogen source on the growth and organic phosphate content of tomato plants was studied. At a very low light intensity ammonium stimulated growth more than nitrate did, at a high light intensity ammonium restricted growth as compared with nitrate. The damaging effect of ammonium depends more on conditions of light than of temperature. The leaf growth ratio and the organic P content ratio for the leaves of the nitrate- and ammonium-fed plants were inversely correlated.