The study compared changes in essential oil content and its thujone ratio in two popular herbs (Artemisia absinthium L. and Salvia officinalis L.), pertaining to plant development and plant organs. Both species were harvested in 2018 at the vegetative, floral budding, flowering and after flowering phases; flowers and leaves were sampled separately. The essential content is always higher in the flowers than in the leaves at the same phenophase in both species we examined. Decreased essential oil content in both organs during the developmental phases was also common to both species. In S. officinalis, both leaf and flower oils showed quantitatively different compositional profiles. During plant development, the main component α-thujone decreased significantly in leaf oils, while both thujone isomers demonstrated statistically stable values in flower oils. In A. absinthium, leaf and flower oils exhibited similar thujone ratios. During plant development, neither of the thujone isomers changed significantly in leaf oils but the ratio of α-thujone increased in the flowers. It was established, that only the distribution and dynamics of total volatiles showed common features in the two species, while the variability of thujone ratios represents differences specific to each target species. For the praxis, in S. officinalis timing of harvest seems to be more important while in A. absinthium the ratio of organs may play a more significant role in reaching lower thujone levels of the drug
