THE FERTILITY OF DAIRY CATTLE IS LOW DURING PERIODS OF HEAT STRESS AND THIS REDUCTION IN REPRODUCTIVE PERFORMANCE HAS A SIGNIFICANT IMPACT ON PRODUCTIVITY AS WELL AS THE COST OF PRODUCTION. EFFECTIVE STRATEGIES TO COMBAT THE HEAT STRESS-INDUCED REDUCTION IN FERTILITY HAVE BEEN SLOW TO DEVELOP BECAUSE OF THE UNIQUE PHYSIOLOGY OF HEAT STRESSED DAIRY CATTLE. INDEED, THE ENDOCRINOLOGY AND METABOLISM OF A HEAT STRESSED DAIRY COW IS UNLIKE ANY OTHER PHYSIOLOGICAL STATE. THE OVERARCHING HYPOTHESIS OF THIS PROJECT IS THAT THE ALTERED ENDOCRINOLOGY AND METABOLISM OF THE HEAT STRESSED DAIRY COW IS A SIGNIFICANT CONTRIBUTOR TO THE OBSERVED REDUCTION IN FERTILITY, AND IS ADDITIVE OR MAYBE EVEN SYNERGISTIC WITH THE DIRECT EFFECTS OF HEAT STRESS ON REPRODUCTIVE COMPONENTS. IF THIS IS INDEED THE CASE, MANAGEMENT STRATEGIES THAT ADDRESS THE ALTERED ENDOCRINOLOGY/METABOLISM CAN BE ADDED TO CURRENT HEAT MITIGATION PROTOCOLS TO SIGNIFICANTLY IMPROVE THE REPRODUCTIVE PERFORMANCE OF HEAT STRESSED DAIRY CATTLE. IN ORDER TO DETERMINE THE RELATIVE CONTRIBUTION OF ALTERED ENDOCRINOLOGY/METABOLISM, SEQUENTIAL TREATMENTS AND ANALYSES WILL BE CONDUCTED DURING THIS PROJECT AND DETRIMENTAL EFFECTS ON INDIVIDUAL COMPONENTS OF THE OVARY WILL BE EXAMINED. THE RESULTS OF THE PLANNED EXPERIMENTS WILL BROADEN THE CURRENT UNDERSTANDING OF HEAT STRESS PHYSIOLOGY, SPECIFICALLY AS IT PERTAINS TO THE LACTATING DAIRY COW AND HER REPRODUCTIVE COMPETENCY. THE ANTICIPATED IMPACT OF THIS PROJECT, AND THE RESULTING KNOWLEDGE IS THAT HEAT MITIGATION STRATEGIES WILL BE EXPANDED TO ADDRESS THE UNIQUE PHYSIOLOGY OF THE LACTATING DAIRY COW, THEREBY IMPROVING PRODUCTIVITY AND REDUCING THE COST OF PRODUCTION DURING PERIODS OF HEAT STRESS.