Cotton (Gossypium hirsutum L.) is a major fiber crop; however, cotton yields are often limited due to its extreme sensitivity to environmental stress such as high temperature and drought. The current project was designed to evaluate the use of the plant growth regulator 1-methylcyclopropene (1-MCP) to alleviate the adverse effect of environmental stress on cotton yields. Three field studies were conducted in Arkansas in 2006 and 2007. An untreated control was compared with 1-MCP at 10 g ai ha-1 applied at first flower (FF), and another applied at FF and again two weeks later. Measurements were made of boll weight, boll number and yield, as well as on plant physiological and biochemical responses. During the period of cotton fruit development, the maximum temperatures for Marianna, AR in 2006 and 2007 were well above the optimum 30oC temperature for cotton growth, indicating that cotton was under heat stress. Plants that received 1-MCP at FF and FF+2 weeks had significantly higher seed cotton and lint yields than the untreated control. This result was possibly due to a significant weight increase in 1-MCP treated bolls located in the middle of the plant canopy as no effect was observed on cotton fruit abscission. Applications of 1-MCP significantly decreased the stress levels of cotton plants as indicated by higher maximum quantum efficiency of Photosystem II and lower activity of the leaf antioxidant glutathione reductase. No 1-MCP effect on protein and malondialdehyde content in leaves was observed. The study showed that foliar application of 1-MCP has the potential to improve yields due to reduced plant stress.