The aim of this study was to determine the effect of manganese(II) on the respiratory activity and cellular metabolism of activated sludge from two different wastewater treatment plants (WWTP). The tested manganese concentrations were: 0.5, 1, 1.5, 2, 4 and 6 mg/L. The effect of this heavy metal on endogenous metabolism was studied by determining the specific endogenous respiration rate (SOURendo), chemical oxygen demand (COD) and measures of enzymatic activity (dehydrogenase activity), while the effect on the biodegradation capacity was determined through the terminal velocity of oxygen consumption (OURexo). Measurements were taken at 30 min, 3 h and 24 h of exposure of the sludge to the heavy metal. The results showed that concentrations below 1 ppm produce an increase in the respiratory activity of both sludges. Increasing the concentration of manganese (>= 2 mg/L) resulted in substantial reductions in respiratory activity, being more significant for long exposure times. Finally, it was observed that activation of the endogenous respiration rate at low concentrations of manganese during the first hours of the test produces greater stabilization of the sludge, as identified by a lower terminal velocity of synthetic wastewater degradation and lower values of COD measured in the supernatant. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.