To determine how much organic nitrogen is lost from lakes during winter by natural processes, we collected water in fall and winter from six small lakes (area, 5–822 hectares) and separated organic matter dissolved in the water withn-butanol into three fractions—yellow organic acids, a white precipitate, and aqueous (nonextractable) organic matter. The nitrogen content of each fraction was measured by ultraviolet photolysis. About 25–30% of the yellow acid and white precipitate fractions were lost from the water column in each of the lakes during winter. More than 80% of the organic nitrogen dissolved in the lake water samples was found in the aqueous fraction. We believe the white precipitate is part of the humin material in lake waters because it was relatively insoluble in acidic and alkaline solutions.