A field study was carried out in order to evaluate different N fertilizers sources in three wheat varieties, considering an Andisol and an Ultisol soils, in the IX Region of the country. The dilution isotopic techniques was used, with AS, 10% at. exc., at rates of 20 kg ha-1 of N. The isotopic parameters such us N in the plant derived from the nitrogen sources, the N in the plant derived from the soil, the fertilizer use efficiencies and the agronomic evaluation between them were determined. The Nddfu (%) was associated to the varieties and to the soils. In the Ultisol, Dalcahue variety had a better behaviour with SS and, in the Andisol, Laurel variety showed an special affinity with U. In the three varieties, the higher % of N derived from the sources it was in the grain, showing Dalcahue variety a better translocation. The fertilizer use efficiency (FUE) and the physiological efficiency, determined according the isotopic parameters, were higher than the values determined according the convencional methodology.