In order to examine the degree of human-induced alteration of the nitrogen and phosphorus cycles at the scale of a tropical watershed of regional dimension, the budgets of these two elements were estimated in the four main sub-basins (Da, Lo, Thao, and Delta) of the Red River system (156 448 km2, Vietnam and China). The four sub-basins differ widely in population density (from 101 inhabitants km-2 in the upstream basins to more than 1000 inhabitants km-2 in the delta), land use, and agricultural practices. In terms of agricultural production, on the one hand, and consumption of food and feed on the other, the upstream sub-basins are autotrophic systems, exporting agricultural goods, while the delta is a heterotrophic system, depending on agricultural goods imports. The budget of the agricultural soils reveals great losses of nitrogen, mostly attributable to denitrification in rice paddy fields and of phosphorus, mostly caused by erosion. The budget of the drainage network shows high retention/elimination of nitrogen (from 62 to 77% in the upstream basins and 59% in the delta), and of phosphorus, with retention rates as high as 80% in the Da and Lo sub-basins which have large reservoirs in their downstream course (Hoa Binh on the Da and Thac Ba on the Lo). The total specific delivery estimated at the outlet of the whole Red River System is 855 kg km-2 yr-1 total N and 325 kg km-2 yr-1 total P. Nitrogen rather than phosphorus seems to be the potential limiting factor of algal growth in the plume of the Red River in Tonkin Bay.