Budgets and dynamics of nitrogen and phosphorus in Lake Donghu were investigated from Oct. 1997 to Sept. 1999. The water residence time was estimated to be 89 days in 1997–1998 and 124 days in 1998–1999. The total external loadings were 53 g N m−2yr−1and 3.2 g P m−2yr−1in 1997–1998, and 42 g N m−2yr−1and 3.1 g P m−2yr−1in 1998–1999. On average. about 80% of nitrogen and phosphorus input was from sewage outlets, while the rest was from land runoff and precipitation. Ammonium ion was the most abundant form of inorganic nitrogen in the sewage. The nutrient output was mainly through water outflow and fish catch. The percentages of nutrients in fish were estimated to be 7.8%-11.2% for nitrogen and 47.6%-49.6% for phosphorus. Lake Donghu has a very high nutrient retention (63% for nitrogen and 79% for phosphorus) mainly due to its closure and long water residence time. Sedimentation is an important nutrient retention mechanism in this lake. Using mass balance method, we estimated that denitrification of Lake Donghu involves about 50% of the retained nitrogen. Lake Donghu is rich in inorganic nitrogen and phosphorus and showed great seasonal variation.