The water quality of the south basin of Long Lake, Minn. was monitored following Ca(NO3)2application in May 1984. The NO2+NO3−Nconcentration was elevated throughout the water column in 1984, but declined to pretreatment levels by 1985. Growing season mean total phosphorus and chlorophyll a concentrations were similar to pretreatment years. The impact of sediment Ca(NO3)2treatment on midsummer water quality in Long Lake was slight because of external loading. Sediment composition and residual NO3−Nmay limit the usefulness of this technique in many lakes.