Biochemical studies conducted under controlled conditions suggested that low concentrations of SO2 which normally do not produce any visual symptoms on the foliage cause injury to forest vegetation by altering pigment (chlorophyll, phaeophytin and chlorophyllide) metabolism, by inhibiting lipid synthesis and activities of various important enzyme systems and by causing ultrastructural disorganization of cellular membranes. In general, aqueous and gaseous SO2 produced results very similar to each other. Plants, when fumigated with SO2 at the ambient air quality standards, exhibited various biochemical responses that can have a deleterious effect on the normal growth and yield of vegetation. However, 24-48 h after transferring the fumigated plants to an SO2-free environment, there was a considerable recovery of such functions.