The effects of Cu2+, Zn2+, Cd2+, Hg2+, Ni2+, Pb2+ and Ag+ on TTC and INT-electron transport system activity are studied and the sensitivities of TTC and INT-electron transport system activity to seven tested heavy metals are compared to evaluate both the parameters for assessing heavy metal inhibition of activated sludge. The median inhibitory concentration (IC50) of seven tested heavy metals measured via TTC-electron transport system activity is less than that measured via INT-electron transport system activity, which indicates that the INT-electron transport system activity is less sensitive to heavy metals than the TTC-electron transport system activity. The ranking of tested heavy metals in order of decreasing toxicity based on TTC-electron transport system activity was Hg2+, Cd2+, Cu2+, Ag+, Zn2+, Ni2+ and Pb2+ , and the ranking based on INT-electron transport system activity was Hg2+, Ag+, Cu2+, Cd2+, Zn2+, Ni2+ and Pb2+.