THE APPLICATION of nitrogen fertilizer has been reported to lower the ascorbic acid content of various crops (Hamner and Maynard, 1942; Maynard and Beeson, 1943-44; Somers and Beeson, 1948, for reviews; also Jones and Parker, 1947). Hamner et al. (1945) reported that the ascorbic acid content of tomatoes from field plots which received fertilizer containing large amounts of nitrogen (2020-10) was about 20 per cent less than from comparable plots which received a fertilizer containing no nitrogen (0-20-10) at the same rate. In sand culture, however, Hamner et al. (1942) failed to find any effect of nitrogen supply upon the ascorbic acid content of tomatoes. Other instances where the amount of nitrogen applied failed to influence significantly the ascorbic acid content of plants are cited by Somers and Beeson (1948). In some cases these different findings may result from the nitrogen source used. Nitrate and ammonium sources of nitrogen supply under suitable conditions may have quite different effects upon the ascorbic acid content of plant tissues (Sideris and Young, 1945; Somers and Kelly, 1950). The following experiment was undertaken to study in more detail the physiological basis for the influence of the amount of nitrogen supplied to tomato plants upon the ascorbic acid content of the ripe tomatoes produced. MATERIALS AND METHODS.-Tomato seed (variety Bonny Best) was planted in sand April 7, 1945, and was watered for 1 week subsequently with Hoagland Solution No. 1 diluted 1:5 (Hoagland and Arnon, 1938). Throughout the rest of the experiment the plants were watered with full strength Hoagland solution except as indicated in table 1. The plants were transplanted to 2 gal. crocks of sand May 6, where they were watered once a day with Hoagland solution and flushed once a week with distilled water. All of the plants were allowed to grow in the greenhouse until June 21, when they were moved outside and were tied to tall stakes. They were pruned to a single stem by frequent removal of the lateral buds. Upon being moved outside the plants were arranged in two blocks of thirty-six plants each. Each block contained six treatments with six plants in each, and the plants were spaced so that they shaded each other very little, if any. The various treatments differed from each other in the amount of nitrate which was supplied to the plants at various periods during the experiment. They are summarized in table 1. The greatest nitrate supply, 15 me./l., was that obtained by using full strength Hoagland solution. The lower rates of nitrate suip-