A variable response to N fertilizer treatments was detected in Thompson Seedless grapevines growing at three locations of the Aconcagua Valley. A positive response was obtained in a low fertility soil, while a negative on was obtained in a high fertility soil. Where plants presented root problems, the response was little with 40 kg N/ha and negative with 160 kg N/ha. Total N determination in leaves was inefficient to detect both the N doses and the differences in growth and productivity. On the other hand, petiole nitrate analysis was very sensible to detect the different levels of N treatments, but annual variation was very high and seriously limited the use of this method. An alternative form of diagnosis based on visual appreciations is suggested. In a locality where a positive response was obtained, total N extraction associated with high productions (21 ton/ha) reached 76 kg N/ha, while in the checks this was only 52 kg N/ha. The increase in total N extraction is mainly due to an increase in total byomass and to a lesser extent to changes in N concentration. This concentration changed little in the pruned material and showed no variation in the fruit as an effect of applied N. K extraction increased from 57 to 76 kg/ha and P from 11 to 15 kg/ha as an affect of applied N. The increase in both cases is due to higher byomass (fruit and vegetation).