Reduction enzymes involved in the denitrification process which are critical for the kinetics of NO 3 −, NO 2 −, N 2O and N 2 are synthesized under anaerobic conditions and repressed by subsequent aerobic conditions. The enzymes are progressively degraded in aerobic soil but are de-repressed by re-introduction of anaerobiosis. Dynamics of these enzymes after different periods of aerobic incubation were investigated for a soil from permanent pasture. Soil anaerobically incubated for 3 days was then aerobically stored at 25°C for 70 days. On days 0, 3, 7, 14, 28, 49 and 70 of storage, soil was amended with a KNO 3 solution and combinations of C 2H 2, glucose and chloramphenicol (to prevent de novo enzyme synthesis). Additionally, on days 28, 49 and 70, soil was first pre-conditioned anaerobically for 3 days at 25°C and then assayed using the same treatments. The production of CO 2 and N 2O and the concentrations of NO 3 − and NO 2 − were then measured at 2, 5, 12, 24, 36 and 48 h in an anaerobic incubation at 25°C. Nitrate reductase concentration appeared to be high relative to the availability of C substrate at the onset of each anaerobic incubation. The reduction of NO 3 − and the production of N 2O increased when glucose was added to the soil although de novo synthesis of the enzyme was inhibited by chloramphenicol. Concentrations of nitrite and nitrous oxide reductase decreased over storage time under aerobic conditions. The proportion of N 2 relative to N 2O formed decreased from 49% on day 0 to 28% on day 3 but was still 21% of gaseous N losses on day 49. No detectable N 2 was formed on day 70. The ratio between the rate of N 2O produced and the rate of CO 2 produced decreased sharply from 1.01 to 0.78 between day 0 and 3 but only gradually declined thereafter. The anaerobic prior incubation of 3 days decreased the concentrations of NO 2 − and the N 2O-to-N 2 ratio, but increased the N 2O-to-CO 2 ratio as compared to the unconditioned soil. Chloramphenicol not only increased CO 2 and N 2O production, but also decreased nitrite reductase and nitrous oxide reductase activity. It was concluded that the duration of aerobiosis affected the concentrations of reduction enzymes involved in the denitrification process and thus the N 2O-to-N 2 ratio when anaerobic conditions were induced. The link between the C mineralization and the reduction of NO 3 − not only depends on the composition of the C substrate but also on the period of preceding anaerobiosis.