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Abstract: Secondary metabolites such as pyrrolizidine alkaloids (PAs) play a crucial part in plant defense. PAs can occur in plants in two forms: tertiary amine (free base) and N-oxide. PA extraction and detection are of great importance for the understanding of the role of PAs as plant defense compounds, as the tertiary PA form is known for its stronger influence on several generalist insects, whereas the N-oxide form is claimed to be less deterrent. We measured PA N-oxides and their reduced tertiary amines by liquid chromatography–tandem mass spectrometry (LC-MS/MS). We show that the occurrence of tertiary PAs is not an artifact of the extraction and detection method. We found up to 50% of tertiary PAs in shoots of Jacobine – chemotype plants of Jacobaea vulgaris. Jacobine and its derivatives (jacoline, jaconine, jacozine and dehydrojaconine) may occur for more than 20% in reduced form in the shoots and more than 10% in the roots. For 22 PAs detected in F2 hybrids (J. vulgaris × Jacobaea aquatica), we calculate the tertiary amine percentage (TA%=the tertiary amine concentration/(tertiary amine concentration+the corresponding N-oxide concentration)×100). We found that the TA% for various PAs was genotype-dependent. Furthermore, TA% for the different PAs were correlated and the highest correlations occurred between PAs which share high structural similarity. [ABSTRACT FROM AUTHOR]