The temporal and vertical distributions of four N species, N03−, NO2−, total ammonia (T-NH3), and free ammonia (NH3), are documented for Onondaga Lake, an urban, polluted, hypereutrophic, dimictic, lake that receives a very high load of T-NH3. Nitrate and NO2− were lost rapidly from the hypolimnion, and T-NH3 accumulated to high concentrations (maximum > 10 mgN L−1), after the onset of anoxia, consistent with the lake's high level of productivity. The concentrations of T-NH3, NH3 and N03− that were maintained in the epilimnion (average concentrations at a depth of 1 m of 2.81, 0.16 and 0.91 mgN L−1, respectively), and concentrations of N02− that developed in the epilimnion (maximum of 0.48 mgN L−1), were high in comparison to levels reported in the literature. These elevated concentrations are largely a result of the extremely high loads of T-NH3, and its precursors, received by the lake. Water quality problems in the lake related to the prevailing high concentrations of N species include potential toxicity effects and severe lake-wide oxygen depletion during fall turnover.