Dairy cattle exposed to summer heat stress (HS) typically exhibit altered metabolic characteristics, including hyperinsulinemia concurrent with hypoglycemia. The reasons for this change in glycemic status and its consequences are currently unknown. This project aimed to examine the relationship between summer HS, blood glucose concentrations, and milk production in primiparous (n=26) and multiparous (n=27) Holstein cows. Glucose was measured in coccygeal vein blood twice weekly using a hand-held glucometer (Abbott Precision Xtra) from May through July. Rectal temperatures were also collected. The days surrounding sample collections were categorized as having no HS (0 h above 68 Thermal Humidity Index [THI]), moderate HS (>0 to 8 h above 68 THI) or severe HS (>8 h above 68 THI). A subset of cows with ≥5 blood glucose measurements spanning different THI categories were categorized as hypoglycemic (n=8 primiparous; n=9 multiparous) or non-hypoglycemic (n=6 primiparous; n=5 multiparous) based upon blood glucose concentrations on no-HS days and severe-HS days (hypoglycemic if sustained reduction on severe-HS days). When analyzed together, milk production did not differ between those categorized as hypoglycemic and non-hypoglycemic. When separated based upon parity, there was a tendency for primiparous hypoglycemic cows to produce more milk than their non- hypoglycemic counterparts (P<0.15). Variability in milk production measured as standard deviation across no-, moderate- and severe-HS days did not differ based upon glycemic status but was greater in multiparous than primiparous cows (P<0.01). Rectal temperatures were greater on severe-HS days than on no- or moderate-HS days (P<0.01) and were positively correlated with the number of h of HS in a day (r=0.45; P<0.01). Neither milk nor glucose differed based upon the category of HS severity. Interestingly, however, glucose was negatively correlated with milk production on the day of measurement (r=-0.24; P<0.01) and day after measurement (r=-0.23; P<0.01). These results are an initial indication that hypoglycemia during HS is related to milk production, particularly in primiparous cows. / Master of Science / Dairy producers lose production and profitability when their dairy cows experience heat stress. Heat stress in dairy cattle during the summer months causes declines in milk production and reproductive performance, which may be at least partially due to the unique metabolic changes observed in heat-stressed dairy cattle. One of these changes is a reduction in blood glucose that has been linked with productivity in previous studies. Measuring blood glucose concentrations may be a useful tool for measuring individual cow responses to heat stress, and ultimately lead to a better understanding of these changes. The work described here aims to explore the relationship between both summer heat stress and blood glucose concentrations in primiparous (calving =1) and multiparous (calving >1) Holstein cows. A total of 53 cows were used, 26 being primiparous and 27 being multiparous. Rectal temperatures, blood glucose measurements and milk production records were collected. A subset of cows were categorized as either hypoglycemic or non-hypoglycemic based on glucose concentrations on non-heat stress and heat stress days. When all cows were analyzed together, milk production did not differ based on glycemic category. When separated based upon parity, there was a tendency for cows that had calved once and were hypoglycemic to produce more milk than their non-hypoglycemic contemporaries. Rectal temperatures were greater on severe-heat stress days than on no- or moderate-heat stress days and were positively correlated with the number of hours of heat stress in a day. Neither the quantity of milk produced, nor blood glucose differed based upon the severity of heat stress. Interestingly, however, blood glucose was negatively correlated with the quantity of milk produced on the day of blood collection and day after blood collection. These Results indicate that measuring blood glucose concentrations could be a useful tool in predicting individual cow's responses to heat and their ability to sustain productivity in summer months.