The effect of transient oxytetracycline (OTC) shock on the stability of anaerobic ammonium oxidation (ANAMMOX) process was evaluated in the present study. The shock test was implemented with 155-1731 mg L-1 OTC, lasting for 1 to 3-fold hydraulic retention times, under the nitrogen loading rate (NLR) of 6.72 and 13.4 kg m(-3) d(-1). The response of the process was divided into shock and recovery stage and the performance under the stress was indicated by stability index and granule characteristic. In the shock period, nitrogen removal rate (NRR) was ranged from 12.1 to 12.1-4.04 kg m(-3) d(-1). The specific ANAMMOX activity (SAA) and heme c content, were respectively reduced by 1.4% and 17.6-29.4%. Foremost, the OTC shock was restorable and the recovery lasted for 4-353 h. The robustness of ANAMMOX process was dependent on OTC level, duration of shock and NLR applied. (C) 2013 Elsevier Ltd. All rights reserved.