Abstract Tanacetum parthenium (L.) Schultz-Bip (feverfew) is among the important medicinal and aromatic plants due to its tryptophan (TRP), serotonin (SER), melatonin (MEL), and parthenolide (PRT) content. In recent studies, have reported TRP, MEL, and (PRT) are effective in the treatment of COVID-19, thus increasing the popularity of feverfew, which is rich in these valuable molecules. This study investigated the possible effects of exogenous foliar applications of methyl jasmonate (MeJA 0.5 mM) and TRP (20 mM) on plant TRP, SER, MEL, and PRT levels. During the pre-flowering period, endogenous TRP was measured as 128.9 µg/mL and endogenous PRT as 1.53% mg/g in the leaves of the control group. During the flowering period, the MEL level was measured as 1.38 µg/mL in the leaves of the TRP application group. In addition, in the pre-flowering period, MeJA-induced increases of 94.51% were determined in DPPH antioxidant activity and the total flavonoid content was 38.76 mg QE/g, whereas the highest total phenolic content of 51.63 mg GAE/g was found in flower samples of the control group. However, neither the developmental periods nor the treatments significantly affected the total phenolic content in the leaves.