Heterotrophic bacteria, yeasts, fungi, plants, and animal breath were investigated as possible sources of N(2)O. Microbes found to produce N(2)O from NO(3) but not consume it were: (i) all of the nitrate-respiring bacteria examined, including strains of Escherichia, Serratia, Klebsiella, Enterobacter, Erwinia, and Bacillus; (ii) one of the assimilatory nitrate-reducing bacteria examined, Azotobacter vinelandii, but not Azotobacter macrocytogenes or Acinetobacter sp.; and (iii) some but not all of the assimilatory nitrate-reducing yeasts and fungi, including strains of Hansenula, Rhodotorula, Aspergillus, Alternaria, and Fusarium. The NO(3)-reducing obligate anaerobe Clostridium KDHS2 did not produce N(2)O. Production of N(2)O occurred only in stationary phase. The nitrate-respiring bacteria produced much more N(2)O than the other organisms, with yields of N(2)O ranging from 3 to 36% of 3.5 mM NO(3). Production of N(2)O was apparently not regulated by ammonium and was not restricted to aerobic or anaerobic conditions. Plants do not appear to produce N(2)O, although N(2)O was found to arise from some damaged plant tops, probably due to microbial growth. Concentrations of N(2)O above the ambient level in the atmosphere were found in human breath and appeared to increase after a meal of high-nitrate food.