Our objective was to study the daily variation in the core-body temperature (CBT) of lactating dairy cows under heat stress. Holstein cows (8 multiparous and 5 primiparous) producing 37.1 ± 2.9 kg/d and at 234 ± 15 DIM were used in the study. Cows were milked twice daily and located in a commercial dry-lot facility in California. The CBT of all cows was synchronously measured at every 10 min throughout 7 d from August 7 to August 13, 2014, using a temperature data logger (iButton, Maxim Integrated) attached to an intravaginal CIDR device. Body condition scores were recorded at device insertion. Environmental temperature was collected at 10 min intervals throughout the experimental period (Average = 29.0°C; Max = 38.9°C; Min = 18.3°C). Individual milk production was recorded daily and extracted from the herd’s management software (Dairy Comp-305). Two cows lost their device and records were not used. Data were analyzed by repeated measures using the GLIMMIX procedure of SAS. In some analysis, cows were categorized into 2 milk production categories (high and low) based on milk production average. Parity, days in milk, and body condition score were not associated with CBT (P > 0.10). In contrast, time of the day and milk production influenced CBT (P < 0.01). Although large individual CBT variations were observed, further analysis comparing cows with high vs. low milk production indicated that maximal CBT was reached later in the afternoon and was greater for high compared with low producing cows (P = 0.04). More importantly, high producing cows remained for a longer period (45.1% of their daily time budget, P < 0.01) with their CBT above the 39.9°C threshold compared with the low producing cows (27.1% of their daily time budget). In conclusion, cows with greater daily milk production seem to be more susceptible to heat stress. Thus, cooling systems should be designed to match milk production requirements.