Similar temporal patterns were found in three mineral soils for the composition of the gaseous products of denitrification following the onset of anaerobic conditions. During the early period of anaerobiosis (0 up to 1 to 3 h), N(2) was the dominant product of denitrification. The NO(3) --> N(2)O activity then increased, but was not accompanied by a corresponding increase in N(2)O-reducing activity. This resulted in a relatively extended period of time (1 to 3 up to 16 to 33 h) during which N(2)O was a major product. Eventually (after 16 to 33 h), an increase in N(2)O-reducing activity occurred without a comparable increase in the N(2)O-producing activity. The increase in the rate of N(2)O reduction did not occur in the presence of chloramphenicol and required the presence of N(2)O or NO(3) during the preceding anaerobic incubation. During the final period (16 to 33, up to 48 h), N(2) was generally the sole product of denitrification, since the rate of N(2)O reduction exceeded the rate of N(2)O production. A similar sequential pattern was also found for a culture of a denitrifying Flavobacterium sp. shifted to anaerobic growth. A staggered synthesis of the enzymes in the denitrification sequence apparently occurred in response to anoxia, which caused first a net production of N(2)O followed by consumption of N(2)O.