Following massive nitrogen (N) fertilization, at rates of 224, 448, and 672 kg N/ha applied in April 1969, nitrate accumulation by species and plant groups on mixed-grass prairie was measured for 3 years. All species and plant groups accumulated Nitrate-N in direct relation to rates of applied N. Two annual forbs accumulated nitrate-N above the 2000 ppm level, which is considered toxic to livestock. In 1970, the first year of residual effect, slimleaf goosefoot contained nitrate-N levels two to three times higher than the potentially toxic level, and in 1971 greenflower pepperweed contained nitrate-N levels slightly above the potentially toxic level. The use of massive rates of N as a range improvement practice should be used with caution unless potentially toxic species are controlled.