Diurnal rectal temperatures of lactating Holstein cows were utilized in a series of studies involving shade modifications with spray and fan in addition to herd data during winter and summer to derive a physiologic index. Physiologic stress degree hours was the product of hours of rectal temperature greater than 39°C. Rectal temperature of cows in shade averaged 39.5°C for 19.6 hours (774.5°C h) compared to 39.3°C for 12 hours (471.6°C h) for shade with spray. Nighttime rectal temperature of cows in both groups was 38.5°C, but milk production declined significantly for cows in shade only. In a follow‐up study, shade modification with spray and fan eliminated hours of rectal temperature above 39°C while rectal temperature averaged 39.4°C for 12 hours for cows in shade (471.8°C h). Rectal temperature of cows in different stages and intensities of milk production was similar during winter. On a record hot day (41.4°C), diurnal pattern of rectal temperature identified high producing cows in early lactation as the most heat sensitive. Milk production declined significantly with more than 16 hours of rectal temperature greater than 39°C.