Estimated at approximately 10 million ha, turfgrass area in the U.S. continues to grow, as once rural/natural landscapes are converted to housing developments, golf courses and other green paces. At the same time, constraints on water and the growing specter of climate change will continue to challenge the ability of turfgrass managers to maintain turfgrass the current high standards. These concerns have motivated turfgrass scientists to improve our understanding of stress tolerance in turfgrasses and to develop methods that will enhance turfgrasses performance during stressful conditions. This thesis focuses on the impact of stress-mediating compounds and Neoptyphodium fungal endophytes on environmental stress (drought, heat, herbivory) tolerance of Kentucky bluegrass and tall fescue. Infection by the symbiotic fungal endophyte Neotyphodium coenophialum (Morgan-Jones & Gams) Glenn, Bacon & Hanlin resulted in lower levels of the antioxidative enzyme catalase (CAT) in tall fescue under field conditions and at elevated temperatures in the environmental chamber. Levels of another antioxidative enzyme peroxidase (POD) were also lower in endophyte-infected plants at elevated temperatures in the environmental chamber. Below-ground herbivory by larvae of the Japanese beetle Popllia japonica Newman (JB) resulted in significantly lower levels of POD under field conditions despite the fact that JB larval densities were only marginally higher in infested plots. Findings indicate that below-ground herbivory and endophyte infection may significantly alter levels of antioxidative enzymes in tall fescue under field conditions, especially at elevated temperatures. While the influence of below-ground herbivory on expression of anti-oxidative enzymes could potentially be exploited in the development of detection and mitigation technologies, observed differences associated with N. coenophialum indicate the need for further research aimed at understanding the potential relationship between these enzymes and the higher tolerance to environmental stress associated with endophyte infection.