Miscible displacement and soil perfusion techniques were used to study the transformations of nitrogen in fractions of anaerobically digested sewage sludge. In miscible displacement experiments the rates of nitrification of NH4+-N of supernates of sludge were 115 μg NO3−-N/g soil/day at a flow rate of 0.17 cm h−1, and 81 μg NO3−-N/g soil/day at the lower flow rate of 0.10 cm h−1. The soil perfusion experiments indicated that only the ammonium-nitrogen of the sludge solids was oxidized to nitrate-nitrogen. The rates of nitrification of sludge were 37 μg NO3−-N/g soil/day for an application of 5.0 cm ha−1 and 15 μg NO3−-N/g soil/day for a sludge application equivalent to 2.5 cm ha−1. The experiments were not of sufficient duration to determine that mineralization of the organic-nitrogen in the digested sludge and subsequent nitrification occurred.