For effective application of methane-dependent denitrification (MDD) in the treatment of wastewater containing NO2- or NH4+, the effect of these inorganic nitrogen compounds on MDD activity needs to be clarified. The MDD activity of sludge acclimatized with CH4 and O-2 was determined with mineral media of different nitrogen-compound compositions in the presence of 0.21 atm CH4 and 0.20 atm O-2. Incubations with media containing only NO2- or two of the three inorganic nitrogen compounds (NO3-+NO2-, NO2-+NH4+ or NH4++NO3-) resulted in MDD activity equal to or higher than that with media containing only NO3-. However, there was no MDD activity in media containing NO2- at 10 degreesC, probably because of serious inhibition of NO2- on methane oxidation. MDD occurred in media containing only NH4+, although the total nitrogen removal efficiency was very low. These results show that NO2- and NH4+, in the presence of NOx-, do not inhibit but rather promote MDD. Consequently, NH4+ does not need to be completely oxidized to NO3- in the nitrification reactor before MDD. However, under psychrophilic conditions, NO2- seriously inhibited MDD. Therefore, the nitrification reactor must not discharge effluent containing NO2- under psychrophilic conditions.