1-Methylcyclopropene (1-MCP) is a biopesticide approved for use in fruit and vegetable by the EPA. The product has been reported to have growth regulating effects by decreasing the effect of ethylene which normally acts as an endogenous stress and senescence phytohormone. 1-MCP may be able to counteract the effects of stress and maintain fruit and seed numbers for increased yield. Field and controlled environment studies were conducted in Arkansas 2005 and 2006 to evaluate the use of 1-MCP to alleviate the adverse effects of high temperature and water deficit on boll set, and thereby reduce year-to-year yield variability, and allow the cotton crop to yield closer to its potential. An untreated control was compared with 1-MCP @ 10 g ai/ha applied at first flower and at first flower and two weeks later. Measurements were made of boll retention, seed set, boll weight, boll number and yield, as well as on plant physiological and biochemical responses. Results indicated increased boll number but no significant effect on seed set or seed weight. There were also effects on fluorescence, polyamines and antioxidant enzyme activity which are being used to explain the effects of 1-MCP on the growth and yield of cotton.