Qualitative properties of Melissa officinalis L. have been studied during five vegetation periods at three localities in Slovak Republic. The results are averages of the factors under investigation. The oil content in individual leaf insertions ranged in the way that the highest amount of the oil was contained in ontogenetically youngest ones. The composition of the oil in stalks and leaves depended also on the ontogenetic phase of plant development, whereas exists a close relationship among the contents of citronellal, neral and geranial.