The immediate and long-term impacts of potassium permanganate (KMnO4) as pre-oxidant on Microcystis aeruginosa and microcystin-LR (MC-LR) release risk were investigated. The cell density and the integrity of M. aeruginosa were determined by a flow cytometry, and typical photosynthetic parameters were measured by a pulse amplitude modulated fluorometer. The photosynthetic parameters were reduced to different degrees, accompanied with slight cytoclasis and complete degradation of extracellular MC-LR immediately after various dosages KMnO4 oxidation (2-20 mg L-1). In a 6-d cultivation following 5 mg L-1 KMnO4 oxidation, the cell density decreased from 3.9 x 10(6) to 0.6 x 10(6) cells mL(-1), and then increased to 0.9 x 10(6) cells mL(-1), while the extracellular MC-LR increased from 0 to 51.2 mu g L-1. In the cultivation after 10 mg L-I KMnO4 treatment, the intracellular MC-LR and cell activity significantly declined, while significant cytoclasis (cell density from 3.8 x 10(6) to 0 cells mL(-1)) and MC-LR release (increase from 0 to 15.2 mu g L-1) were observed. The photosynthetic parameters were found to be useful tools to predict the recovery tendency of M. aeruginosa cells, and the MC-LR release risk should be considered during KMnO4 pre-oxidation in water-treatment plants. (C) 2012 Elsevier B.V. All rights reserved.