Several interlaboratory exercises were organised within the framework of European FP6 project NORMAN. Among others, non-steroidal anti-inflammatory drugs were investigated in different aqueous samples in two sequential ring studies. The aim of both studies was to evaluate the state-of-art in Europe and to determine possible sources of variation, while also attempting to diminish them. In the present paper we discuss the results of the 2nd Interlaboratory study, while the results of 1st round were presented before. The main scope of the 1st exercise organised within NORMAN project was to assess the laboratory proficiency regardless of the analytical method applied, to evaluate the stability of the target compounds during sample storage, and to define possible sources of variation during sample shipment, storage and analysis. In the 2nd round we primarily aimed to diminish these sources of variation by applying two predetermined analytical protocols based on liquid chromatography-mass spectrometry or gas chromatography-mass spectrometry. The two analytical protocols were compared in terms of their ability to determine individual analytes in matrices of different complexity, i.e. tap water, river water and wastewater. Furthermore, the 2nd exercise addressed also the filtration and compared the influence of different filter material categories on the analysis of non-steroidal anti-inflammatory drugs. Results presented herein evaluate laboratory performance using z-score, bias, proximity and Youden plots. Overall, the laboratory performances were found to be satisfactory for determining NSAIDs in aqueous samples. The two analytical protocols, LC-MS and GC-MS, are assessed according to their sensitivity and measurement uncertainty, where the GC-MS proved superior for the analysis of Ibuprofen, Ketoprofen and Naproxen in matrices with higher complexity. Finally, neither the filtration itself, nor the filter materials were shown to significantly affect the determination of NSAIDs.