Lowering the pH of cattle slurry with HNO3 was used to reduce NH3 volatilization during storage and after application. Incubation studies were carried out to examine possible NO3 losses and N2O emission from HNO3 treated slurry during storage. Batches of cattle slurry were treated with various amounts of HNO3 to obtain a pH range of 6.0 to 3.0. The slurries were stirred once or twice a week and stored for 6 months at 15 degrees C. Changes in pH, Eh, NO3- and NH4 concn, and emissions of N2O, CO2 and CH4 were monitored. The loss of NO3- and the emission of N2O were related to slurry pH, being lowest at low pH. Cumulative loss of NO3- ranged from ~40 mmol/kg for slurries of target pH 5.0 to ~400 mmol/kg for slurries of target pH 6.0. Homogenization of the slurries via stirring and addition of H2O2 decreased NO3- loss and H consumption. The strong pH dependence of NO3- loss, the production of N2O and the stoichiometry of H consumption and NO3- loss indicated that the loss of NO3- was mainly due to microbiological denitrification. Similar N2O production rates in the presence and absence of C2H2 indicated that nitrification was not an important source of N2O. It is concluded that lowering the pH to values 4.5 as well as regular and complete homogenization of the slurry via stirring are important for the success of the on-farm treatment of slurry with HNO3.