The specific oxygen uptake rate (SOUR), specific nitrification rate (SNR) and specific denitrification rate (SDNR) of an anoxic-oxic activated sludge process fed with zinc-added synthetic wastewaters were investigated. Two different characteristics of synthetic wastewaters were used, i.e., 500 mg/l COD, 40 mg/l TKN and 10 mg/l P (representing normal COD load) for Model A while 3500 mg/l COD, 175 mg/l TKN and 25 mg/l P (representing high COD load) for Model B. The zinc doses varied from 0 (control) to 10, 25, 35 and 50 mg/l. When the two systems reached steady states, they were further shocked with 300 mg/l zinc for 4 consecutive days before returning to their initial conditions. The SRT and F/M ratio of both models were 10 days and 0.26-0.47 day−1, respectively. The endogenous SOURs of both models were not much affected by the increase of zinc concentration. They were about 7.5 to 10 and 9.4 to 11.5 mg O2/g MLSS-hr, for Models A and B, respectively. In Model A, as the zinc increased from 0 to 50 mg/l, the SNRs dropped from 4.0 to 1.4 mg NH4+-N /g MLSS-hr whereas the initial SDNRs fell from 19.6 to 5.3 mg NO3− /g MLSS-hr. Meanwhile, the SNRs of Model B were relatively constant (1.5-1.8 mg NH4+-N /g MLSS-hr) while the initial SDNRs dropped from 16.2 to 8.3 mg NO3− /g MLSS-hr. That is, under high COD load conditions, the zinc dose applied here did not significantly affect the carbon removing heterotrophs and nitrifiers while a slight effect was seen on the denitrifiers and significant retardation was observed for both nitrifiers and denitrifiers in case of normal COD load. During the shock period, the SNRs of Model A dropped to 0.67 to 1.26 mg NH4+-N /g MLSS-hr whereas the initial SDNRs decreased drastically to 1.5 to 3.0 mg NO3− /g MLSS-hr. The impact from the zinc shock in such circumstances was obviously higher on the denitrifiers than on the nitrifiers. In Model B, the SNRs were 0.77 to 1.5 mg NH4+-N /g MLSS-hr and the initial SDNRs were 2.9 to 6.18 mg NO3− /g MLSS-hr. Not much effect on nitrifiers was evident in this case. For Model A, the recoverability of the heterotrophs and the nitrifiers was not so good, while that of the denitrifiers was quite satisfactory. However for Model B, those recuperation abilities were comparable for all three organisms. The data therefore suggested that there may be some differences in the species domain between the carbon removing microorganisms and the denitrifiers, however, further investigations for confirmation are required.