Degradation efficiency, mechanism and intermediates' toxicity of aminopyrine (an analgesic and antipyretic drug) upon ozonation were investigated under different oxidation approaches. Results showed that hydroxyl radical pattern (O-3 with H2O2 addition) had the highest removal efficiency in aminopyrine, UV254 and especially DOC. A total of 21 intermediates from aminopyrine oxidation were assessed by UPLC-Q-TOF-MS, which indicated that aminopyrine was degraded mainly from three pathways. The pyrazole ring break pathway consisted of pyrazole ring opening, demethylation, functional group loss, hydroxylation and substitution, demonstrating the major pathway for aminopyrine oxidation. The demethylation at N-6 position pathway was composed of demethylation at the N-6 position and further substitution at the C-4 position, principally occurred during the aminopyrine ozonation with H2O2 addition. The dephenylization pathway was proved by only one intermediate (P21) during the aminopyrine ozonation without addition. Besides, lots of hydroxylated and di-hydroxylated intermediates were detected primarily during the oxidation without addition and with H2O2 addition. The toxicity of these intermediates by EPA TEST showed that some of them were intended to be more toxic than aminopyrine. Further test from the toxicity of oxidized mixtures to the bioluminescent marine bacterium Vibrio fischeri demonstrated the samples could lead to the accumulation of toxic transformation products, especially for those by the oxidation with HCO3- addition. Finally, an O-3-BAC system was applied to treat the raw water spiked with aminopyrine. Although three of the by-products could be detected during aminopyrine ozonation, they were expected to be easily removed by BAC adsorption. (C) 2015 Elsevier B.V. All rights reserved.