In spring, with the appearance of new foliage, the aromatic content of the air as measured with the gas chromatograph does not increase much, and the air is essentially free of terpenes such as a-pinene. Even when plastic bags are put around young shoots of Aster, no terpenes are detectable inside the bag until the leaves are crushed, when large amounts of terpenes are immediately released. Soon the a-pinene content of the plastic bag with the crushed Aster leaves reverts to zero, and rises again upon renewed crushing. Similar experiences were found with other young plants such as Monarda. This indicates that terpenes are produced at all times but are not released until the leaves become older or when their cells die. Conclusion.-Using a sensitive gas chromatograph mounted in a mobile trailer laboratory, the presence in air of many organic compounds in molecularly disperse state can easily be measured. The hydrogen flame detector, which responds only to organic molecules, has a sensitivity of better than 1: 109, using 5-cc air samples. Whereas in cities gasoline and other man-produced organic vapors constitute the bulk of the organic volatiles in the air, in the countryside, away from highways and human activities, plant products predominate. Among these, aand fl-pinene, myrcene, and isoprene were identified. Their concentration depends on meteorological conditions and density and activity of plant cover; during summer usually more than 10-8 organic volatiles occur in country or forest air; during winter this decreases to 2 X 10-9. After one or more dark rainy days plants release less terpene. Upon the death of cells large amounts of terpenes are released, explaining the aromaticity of drying hay or of forests during autumn. High concentrations of terpenes in the air were associated with the dying of leaves in autumn, and with the mowing of meadows. A world production for plant volatiles released to the atmosphere is estimated to be 438 X 106 tons per year.