A study is reported about sensitivity to total nitrogen and nitrate changes taking place in corn leaf parts (basal, median and top parts) and tissue (blades or midribs). Also a sampling procedure is described. Samples were collected from one NPK fertilizer experiment of the 3³ factorial design, the plants showing a definite response to nitrogen fertilizer. Fertilizers were supplied at the rates of 0,60 and 120 kg per hectare of each N, P2O5, and K2O. Nitrogen was supplied as ammonium sulphate; one fourth was given at planting time, the remaining topdressed when the plants were 40-50 days old. It is suggested that leaves should be sampled about 9 weeks after planting, when tassel is just emerging or a little earlier. The position of the leaf on the plant is indicated by Kuijper system, as leaf of position +4. Figure 1 shows the right position and the proper part of the leaf to be selected for analysis. Sufficient information is available to utilize the procedure outlined as a method to, diagnose nitrogen needs: A) at 60 to 70 days after planting the nitrogen absorption rate becomes more pronounced for corn plants; B) the data of chemical analysis, as obtained, gave a good indication of nitrogen status of the crop; and C) at that age it should still be possible to correct nitrogen deficiency, by adding fertilizers. The results indicated the basal, third of the leaf as the best part for diagnosing the nitrogen status, the midribs being analysed for nitrate-nitrogen and the remaining portion of the sample analysed for total nitrogen. Plants deficient in nitrogen gave low yields of grain. Mean values for NO3-N concentrations of midribs were 341 ppm for deficient plants or untreated plants, as compared with 1410 and 2119 ppm tor those receiving 60 and 120 kg N per hectare, respectively. Analysis for total N in the leaves showed mean values of 2.12, 2.79 and 3.11 per cent, respectively for the three levels of fertilizer. Thus the data indicate that nitrate-nitrogen values provide a more sensitive index of the nitrogen status of the corn plant than total N values. Phosphate fertilization had a marked influence on the levels of nitrogen in the leaves, mainly on nitrate-nitrogen content. These effects were negative, as can be seen in table 4. The greater coefficient of variation for nitrate contents also appearing in table 4 is probably due to a greater dependence of this form of nitrogen on external growth factors, such as temperature, humidity, sunlight, etc. In order to keep sampling errors to a minimum it is suggested to control closely the time of day at which samples are taken.