Monitoring thermal status using a reliable, practical method is essential for proper cattle management during periods of high heat load. Our objective was to determine whether accelerometer-based tags, which measure heavy breathing and other behaviors on a minute-by-minute basis, accurately reflect thermal status. We hypothesized that the tags would mirror core body temperature as recorded by continuous intravaginal loggers, and that response to a cooling intervention would also be captured. Data were collected during a 3-d period from 10 late dry and 20 high-producing cows at peak lactation, each fitted with an intravaginal temperature logger and tag attached to a collar. The relationship between vaginal temperature and proportion of cows recorded as breathing heavily averaged over the three 24-h periods was described separately for each group, and during a 1-h block capturing 2 intensive cooling sessions in lactating cows. Finally, the log of the odds of a cow breathing heavily in each minute relative to vaginal temperature in cows before cooling was analyzed using a linear mixed model. The proportion of cows breathing heavily in a group mirrors changes in vaginal temperature in both dry and lactating cows. In contrast to the dry cows, lactating cows suffered from excessive heat load to a much greater degree. Cooling intensively with showers and fans effectively mitigates the effects of excessive heat load, as reflected by response in vaginal temperature and proportion of cows breathing heavily. The probability of heavy breathing increases by approximately 10% for each 0.5°C rise in vaginal temperature above 39.0°C. In conclusion, accelerometer-based technology that automatically records heavy breathing provides a practical means of continuously assessing heat load status of dairy cows, whether lactating or dry, and their response to cooling intervention. This has the potential to optimize both cow and resource management during periods of high heat load.