(1). Continuous body temperature records from dairy cows for 46 days of summer and contemporary data for climate temperature humidity index (THI) were analysed. (2). A large component of the body temperature data was not explained by changes in THI. (3). Outside a normal range of values (above 72 for THI and 39.05 °C for body temperature), there was evidence of a causal relationship with a time delay of about 120 minutes. (4). Both variables had a prominent circadian component, but these were more likely to be due to common and/or independent causes than to any direct relationship.