The genus Teucrium (Lamiaceae) is comprised of about 340 species widespread over the world. In the Flora Iranica, this genus is represented by 12 species [1]. Various species of the genus Teucrium are used as antiseptic, antipyretic, antiinflammatory, antispasmodic, antinociceptive, anti-rheumatic, anthelmintic, diuretic, hypoglycemic, diaphoretic, and tonic in folk medicine [2]. Phytochemically, several diterpenoids, diterpenelactones, triterpenoids, flavonoids, and minor phenolic compounds have already been isolated from this plant and analyzed [3–7]. A literature survey on the essential oil composition of some Teucrium species revealed that in T. melissoides, T. polium subsp. capitatum, T. haenseleri, and T. capitatum, monoterpenes such as α-pinene, β-pinene, p-cymene, and limonene were the main constituents. Also in the oils of T. oriental L. var. oriental, T. fruticans, T. abutiloides, T. salviastrum, T. turredanum, and T. betonicum, sesquiterpenoids such as β-caryophyllene, germacrene D, and α-humulene were reported to be the major components [8–15]. In this paper the essential oil composition of Teucrium hyrcanicum, which is an endemic plant related to the Caspian sea area, is reported for the first time [16]. Thirty-six components were identified in the oil of T. hyrcanicum representing 93.1% of the oil, and also forty-two components were characterized in the oil of T. chamaedrys ssp. chamaedrys representing 94.1% of the oil. The identified components with their percentages are given in Table 1. These components are listed in order of their elution from the DB-5 column. As it is shown, the oil of T. hyrcanicum consists of oxygenated monoterpenes (16.3%), sesquiterpene hydrocarbons (32.3%), and oxygenated sequiterpenes (24.9%). Hexahydrofarnesyl acetone (12.7%), linalool (11.7%) and E-β-farnesene (10.7%), dihydroedulane (8.6%), and ar-curcumene (8.5%) were the main constituents in this oil. As a result, in the oil of T. hyrcanicum, sesquiterpenes (61.7%) predominated over monoterpenes (16.3%). In contrast, the oil of T. chamaedrys ssp. chamaedrys is rich in sesquiterpenes (63.8%), with α-muurolene (15.3%), β-caryophyllene (15.0%), and Z-β-farnesene (7.6%) as the major components. Among the identified monoterpenes (26.5%) in this oil, α-pinene (7.9%), β-pinene (5.9%), and limonene (5.1%) are the most abundant. The composition of the essential oil of T. chamaedrys (subspecies not referred to) has been reported in the literature. Some differences in the quantity and quality of the oil composition may be due to different subspecies, chemotype, and geographic and climatic factors. For example, in two of them, germacrene D and β-pinene were reported as the main constituents [17, 18].