A steady-state heat transfer model by Nelson and Janni (2016) was used to model thermoregulatory responses of lactating Holstein cows to heat stress. The model describes how dairy cows can manage to maintain thermal balance by adjusting tissue insulation, respiration rate, sweat rate and body temperature when challenged with warm and hot weather conditions (ex. temperature, humidity, solar gain and air velocity). The model was used to assess the impact of changing dry-bulb and dew-point temperatures, mean radiant temperatures when shaded, solar heat gain, cow production level and exposed surface area on a cow-s respiration rate, body temperature and sweat rate. Model respiration rate and body temperature results were similar but not identical to the heat stress categories described by Renaudeau et al. (2012). Model results were consistent with common recommendations for mitigating heat stress in lactating cows. Providing shade and increasing air velocities at cow level reduced respiration rates and body temperatures. Model results also demonstrated that reduced mean radiant temperatures reduced heat stress. Model results aligned with research that higher producing cows and cows crowded together with less exposed surface area for heat dissipation experience more heat stress. The model was used to compare environmental conditions cows on pasture experienced during two heat stress events in 2015. The lactating cow thermoregulation model results demonstrated that many environmental factors, more than just dry-bulb temperature and relative humidity, impact the ability of cows to manage heat stressful conditions.