Abstract: Changes of levels of growth inhibiting substances under drought, salinity, chilling and heat stress in different types of plants were investigated. The presence of ABA in leaf extracts of several plants was identified. Levels of ABA and other inhibitors were increased under stress to different extents, depending on species of the plant and the nature of the stress. The inhibitor contents of two xerophytes, Hedysarum scoparium and Haloxylon ammoderdron, from desert region were also increased by 11.6-fold and 5.2-fold respectively when subjected to water stress, whereas that of mesophyte Robinia pseudoacacia increased 4.5-fold. Correlation between inhibitor level and drought resistance of different wheat cultivars was also noted, The resistant cv. “Hongnong No. 1” accumulated 2–3-fold more inhibitors than the nonresistant cv. “Gansu No.8” when subjected to water or saline stress. The role of growth inhibitors is the adaptation of plants to unfavorable environments was discussed.