Nitrogen is an important ecological factor in tuber yield and tuber quality of processing potato. This work reports the effect of five nitrogen rates on potato, cv."Hermes", grown in Tagus Valley. The nitrogen treatments consisted of applying 0 (N0), 80 (N80), 160 (N160), 240 (N240) and 320 (N320) kg N/ha, as ENTEC® 26, a stabilized fertilizer with a nitrification inhibitor. The field trial was arranged in a completely randomized design with 4 replications. Groundcover percentage by crop canopy was recorded weekly until full coverage of the ground by using a grid. In the treatments N0 and N160, plants were collected biweekly for evaluation of canopy growth performances. At the end of the growing season, data on tuber yield, tuber size, number of tubers, specific weight of tubers, dry matter percentage and nitrogen content in tubers were also recorded. The nitrogen fertilization increased the size of the photosynthetic apparatus, through secondary ramifications. Nitrogen fertilization also increased the tuber yield, although in small amounts from 160 kg N/ha. Tuber size was also influenced by N rate. [ABSTRACT FROM AUTHOR]