Diffuse nitrogen pollution caused by irrigated agriculture is quantified for the Pocochay stream located in the Aconcagua watershed in the Valparaiso region, Chile. The study area has an extent of 93 km(2) with 36 km(23) under irrigation. It is representative for the central part of Chile where intensive irrigation is prevailing and predominantly taking place in the alluvial plains of river systems with water sources from the Andes Mountains. Water quality and discharge were monitored biweekly at several points of the Pocochay main stream from December 2003 to June 2006. The study was supported by an agricultural survey and remote imagery interpretation in order to retrieve data on cropping pattern and actual fertilizer uses in the sub-watersheds. Nitrate - N concentration in the Pocochay stream ranged from 1.3 to 10.1 mg L-1 with a mean of 4.8 mg L-1. Other nitrogen species showed very low concentrations. At average, around 52 kg N ha(-1) a(-1) are drained from the study area as compared to 167 kg N ha(-1) a(-1) applied as fertilizer to the fields. Even though nitrate concentrations in the Pocochay do not reach alarming levels yet, the impact of irrigated agriculture on surface water quality is obvious and makes a case for introducing best management practices to avoid aggravation of contamination.