a total of 63 compounds were identified in hydrodistillates from flowers and leaves of vervain plants [aloysia triphylla (l'herit.) britton] grown in portugal. the three main essential oil compounds present either in flowers or in leaves were geranial (26.8-38.3%), neral (20.8-29.6%), and limonene (5.7-20.6%). 1-octene, 1-octen-3-ol, p-cymene, (z)--ocimene, and trans-carveol, identified in the flower oil, were not found in the leaf oil of the same plant. however, p-cymene and trans-carveol were found in the leaf oils of young cultivated plants. on the other hand, -citronellene, -pinene, neryl acetate, and trans-calamenene, present in the leaf oils, were not found in the flower oils. the main compound groups and percentages found in oils were monoterpene hydrocarbons (29.9 and 22.3%), oxygen-containing monoterpenes (56.9 and 59.0%), sesquiterpene hydrocarbons (6.5 and 6.8%), and oxygen-containing sesquiterpenes (3.0 and 4.8%) (flowers and leaves, respectively). the percentage of the total oxygen-containing monoterpenes, especially aldehydes, in the leaf oils of young cultivated plants, increased from july to december in inverse correlation with that of monoterpene hydrocarbons. the comparison with previous studies performed by other authors points to a significant variation in the chemical composition of vervain oil depending from the origin of the plants.