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In this study, a combined targeted/untargeted UHPLC-ESI-QTOF-MS/MS method for the targeted quantitative analysis of the primary platelet lipid mediators thromboxane B2 (TXB2), 12S-hydroxy-5Z,8E,10E-heptadecatrienoic acid (HHT) and its oxidation product 12-keto-5Z,8E,10E-heptadecatrienoic acid (KHT) was developed, which allowed simultaneous untargeted profiling for the detection of other lipid biomarkers such as other oxylipins and fatty acids (FAs) in platelet releasates. A general procedure for the synthesis of keto-analogs from hydroxylated polyunsaturated FAs (PUFAs) using Dess-Martin periodinane oxidation reagent was proposed for the preparation of KHT standard. MS detection was performed in data independent acquisition (DIA) mode with sequential window acquisition of all theoretical fragment ion mass spectra (SWATH) in the range of 50–500 Da with variable window sizes. The LC-MS/MS assay was validated for the targeted analytes and applied for analysis of supernatants derived from resting platelets and from platelets treated with thrombin. The targeted analytes KHT, HHT and TXB2 were found at highly elevated levels in the activated platelet releasates. On average, 13 ± 7, 15 ± 9, and 0.6 ± 0.2 attomols per platelet were released upon thrombin-activation. Furthermore, the simultaneous untargeted profiling (n = 8 in each group) revealed that these oxylipins are released with a pool of other (significantly upregulated) oxidized (12-HETE, 12-HEPE) and non-oxidized PUFAs. All these compounds can be considered additional biomarkers of platelet activation complementing the primary platelet activation marker thromboxane B2. The other lipids may support platelet activation or trigger other biological actions with some potential implications in thromboinflammation.