Hazelnuts and their skins are important sources of health-promoting compounds. In this study, serotonin, a neuroactive compound, and its precursor tryptophan in 14 hazelnuts, almost all hazelnut varieties in Turkey, in two consecutive harvest years were investigated. Serotonin content of hazelnut skins and the effect of roasting on the serotonin content of hazelnuts were also revealed for the first time. Serotonin and free tryptophan concentrations were determined after water extraction while an alkaline hydrolysis was performed for bound tryptophan before UPLC-MS/MS analysis. Free tryptophan ranged from 42 to 127 mu g/g and bound tryptophan was between 3670 and 8904 pg/g in hazelnut varieties. Serotonin content of hazelnut varieties was ranged from 0.1 mu g/g (Kalinkara) to 2.0 mu g/g (Palaz). Significant differences (p < 0.05) were found in free, bound tryptophan, and serotonin contents of most of the hazelnut varieties depending on the harvest year. Mean serotonin content of hazelnut skins was 4 times higher than hazelnuts. Moreover, it was found that roasted hazelnuts were as valuable as raw hazelnuts in terms of their serotonin content as no significant differences (p > 0.05) were observed after roasting at 150 degrees C for 30 min.