Authors work with sweet Basil (Lamiaceae), an ancient plant, could be raising in Hungary. The plant material was collected from cultivation during different states of the ontogeny mostly in the flowering (early, full, late) and productive periods. Water-steam distillation and gas chromatographic (GC) techniques were used for essential oil investigation. The polyphenol, tannin and flavonoid content were measured by spectrophotometry and detected by TLC, HPLC. Results of our investigations show that the essential oil content increases during the ontogeny till the full flowering state. The composition fluctuate mildly. In the early and full flowering stages the characteristic components are the linalool monoter-pene, the aromatic mefhyl-chavicol and two sesquiterpene compounds (6-cadinol and/or a-bulnesene). In late flowering stage amount of linalool decreases suddenly, and at the same time the sesquiterpene content reached own maximum. The flavonoid glycosides begin to form in plant from budding stage, and increases during flowering period. Finally on the basis of change of active substances, phytomass and volatile oil production, during the vegetation period, authors provide some important suggestions for the favourable harvesting time of Basil.