<p>The present study was undertaken to determine the threshold level of heat stress manifestation on test day milk production traits and to estimate the genetic parameters for effects of heat stress on these traits in Jersey crossbred cows, maintained at the National Dairy Research Institute, Kalyani, West Bengal, India. Test day production traits included milk yield (MY), fat% (FP), fat yield (FY), protein% (PP), protein yield (PY), 4% fat corrected milk yield (4% FCM) and energy corrected milk yield (ECM). The heat stress was assessed by Temperature-Humidity Index (THI), calculated using meteorological information obtained from NASA POWER database. A repeatability test-day reaction norm (RN) model with the heat stress function was used for genetic evaluation of heat stress of animals. The study showed a significant (<i>P</i> &lt; 0.05) decline in all studied traits except for FP per unit increase in heat load in Jersey crossbred cows. Heritability decreased for all the production traits along the THI trajectory, except for PP, for which a U-shaped curve was obtained. However, the permanent environmental effect increased along the THI trajectory with exception in PP, where a decline was observed. The correlations between general additive and heat stress additive were negative for all the production traits and ranged from medium to high (-0.35 to -0.95). The genetic correlation between permanent environmental and heat stress permanent environmental ranged from -0.22 to -0.53. In this study, antagonistic genetic relationship between cows’ ability to produce under thermo-neutral vs. thermo-stress conditions, indicates that cows with higher production potential will tend to have larger decays in production in heat stress condition. Hence, breeding programs should incorporate heat tolerance by using reaction norm models or selecting animals with flatter production declines across rising THI for Jersey crossbred cattle in India.</p>