Previous work by many investigators has suggested that the process of nitrogen assimilation in the plantpasses through a step involving the reduction of nitrates to nitrites, and finally to fully reduced nitrogen carriers which function directly in the synthesis of amino acids and proteins. Eckerson (4) advanced the theory that this process of reduction is governed by an enzyme, the so-called reducase, or reductase, which catalyzes the reduction of nitrates to nitrites. Conditions necessary for this reduction to take place are, among others, the presence of phosphorus, calcium, and potassium. Deficiency of phosphorus during the growth of tomato plants leads to an abnormal? accumulation of nitrates in the tissues, caused, as suggested by Eckerson (4), by the inability of the plant to utilize the nitrates because of a breakdown in its reduction mechanism. The plant is thus exhibiting a type of deficiency caused both by nitrogen and phosphorus starvation. Evidence for the foregoing theory is based mainly on experiments designed to measure the nitrate reducing ability of plant juice, which appears to be impaired in the case of phosphorus deficiency. Further evidence is based on the known fact that nitrates do accumulate in rather large amounts in the tissues of some plants suffering from phosphorus starvation. More recently the problem has been attacked from a different angle. Skok (12) studied the effect of the form of available nitrogen on the development of calcium deficiency symptoms in the bean plant and has shown that in plants supplied with urea instead of nitrate nitrogen the development of deficiency symptoms due to lack of calcium is noticeably delayed. This work is interpreted to indicate that, since nitrogen has been supplied in a form that is already reduced, the necessity for nitrate reduction has been removed, and the plants are exhibiting true calcium deficiency. These experiments tend to confirm the essential r?le of calcium in nitrate reduction. The present work is an attempt to apply and extend the same type of reasoning to a study of phosphorus deficiency in the tomato plant, in order to determine more clearly if phosphorus does play an essential r?le in the assimilation of nitrates, and if so, to obtain information on what stage of the process is affected by a deficiency of phosphorus. The mere fact that plants utilizing urea as a source of nitrogen show delayed appearance of deficiency symptoms is not adequate proof of the essential nature of phosphorus for the reduction of nitrates for, as previously pointed out (2), these plants absorb phosphorus much more rapidly from the nutrient solu1 Journal Paper no. 137 of the Purdue Agricultural Experiment Station.