A dynamic two-layer mass balance model for total nitrogen (N) and various species of N is developed and tested for Onondaga Lake, NY, a system in which high concentrations of total ammonia (T-NH3) prevail. The model simulates concentrations of T-NH3, nitrate plus nitrite, particulate organic N (PON), dissolved organic N, total Kjeldahl N, and total N. Processes of the N cycle accommodated include net phytoplankton growth, nitrification, denitrification, ammonification, decay of PON, volatilization of free ammonia, sediment release of T-NH3, settling of PON, and vertical mixing-based exchange between the layers. Model testing is supported by a comprehensive monitoring database of lake concentrations of the N species, and tributary and waste discharge concentrations for calculation of loads. Model credibility is enhanced by the independent determination of several important model coefficients based on field and laboratory experiments, which greatly reduces the role of calibration. Model verification is established by the successful simulation of the distinctly different distributions of various N species documented for the lake in 1989 and 1990. The model is particularly successful in simulating T-NH3concentrations in the upper waters, and is thus appropriate to evaluate various related remediation strategies for the lake.