This study aimed to quantify dairy cows' perceptions of heat (in)tolerance using a trade-off procedure: we assessed the ambient conditions at which cows would give up a highly-valued activity (resting) in order to obtain shade. Under a range of naturally varying ambient thermal conditions, one group of eight cows (Experimental) were provided with a mutually-exclusive choice of Resting or Shading areas for 75 min. Each cow was tested at each of three levels of resting deprivation (0, 12, 24 h). Individual cows in a second group of eight animals (Control) were provided with 75 min access to a lying area only, at each of the resting deprivations. Measures of ambient environmental parameters (temperature, humidity, solar radiation, wind speed, black globe and ground temperature) were recorded in the open and in the choice areas. For Experimental cows, the time spent in each area, switches between areas and total time resting were recorded, whereas for Controls total time resting was recorded. In addition, internal body temperature (Tb) and respiration rates (RR) pre-and post-testing, and external coat temperature (Tc) were recorded for all animals. The Shaded area was significantly cooler than the Resting area (t-test, p<0.001). The proportion of time resting increased with increasing duration of prior rest deprivation (REML, p<0.001), and the proportion of time in shade (in choice tests) decreased with increasing rest deprivation (REML, p<0.05). In the choice test, 24 h resting-deprived cows decreased time resting (REML, P<0.05) when ambient temperatures exceeded 29°C (when RR, Tb and Tc were 70bpm, 38.5°C and 40.0°C, respectively). Since rest is a highly-valued activity to 24 h lying deprived cows, the results indicate that shading becomes critical at temperatures approaching 30°C.