For investigation of the effects of aeration on nitrite-and nitrate-transforming activities of various heterotrophic bacteria, a series of coefficients of the oxygen absorption rate (K-d, 8-99 x 10(-7) mol/ml.min.atm) in 500-ml shaking flasks were determined by varying plug types and culture volumes. Bacillus badius I-73, which neither shows denitrification activity nor utilize nitrate as a nitrogen source, consumed nitrite and accumulated nitrate at all K-d values at which experiments were conducted. In B. subtilis I-41, which does show denitrification activity, the manner of nitrite and nitrate conversion was influenced by the culture time and K-d, and the direction of conversion was changed from reduction to oxidation, as the K-d of the culture increased. Pseudomonas pavonaceae, another denitrification-positive strain, metabolized both nitrite and nitrate to more reduced compounds at low K-d, and the direction of conversion changed from reduction to oxidation at K-d=20 x 10(-7) mol/ml.min.atm. Such switching behavior was also observed when P. pavonaceae was cultured continuously during variation of the aeration conditions with supply of pure oxygen. Many other denitrification-positive strains behaved similarly to P. pavonaceae, and showed their own critical K-d, the point at which the direction of nitrite metabolism changed. The results of intact-cell reaction experiments indicate that this switching might be caused by inhibition and repression of nitrite-reducing activity, and by stimulation of nitrite-oxidizing activity by oxygen.