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Although taking body temperature by rectal thermometer is the method most commonly used to identify sick cows in the postpartum period, no data on the repeatability of this measure are available. The overall objective of this study was to evaluate variability of rectal temperatures in dairy cows considering different factors (intra- and interinvestigator repeatability, different thermometers, penetration depth into the rectum, and defecation). High coefficients of correlation (r=0.98) and small differences between values of rectal temperatures (observer A=39.3+/-1.0 degrees C and observer B=39.4+/-1.0 degrees C) provide evidence that rectal temperature was a repeatable measure in dairy cows. Testing was carried out using 4 different digital thermometers: GLA M750 (GLA Agricultural Electronics, San Luis Obispo, CA), MTI8101 (SES Scala Electronics, Stahnsdorf, Germany), MT1831 (Microlife AG, Widnau, Switzerland) and Domotherm TH1 (Uebe Medical GmbH, Wertheim, Germany). Thermometers were inserted into the rectum to a certain depth (GLA M750 and MTI8101=11.5cm; MT1831=8.4cm; Domotherm TH1=7.7cm) and a measure was finished when a visual or acoustic signal was emitted by the thermometer. The measures could be influenced by the procedure itself (up to 0.5 degrees C), type of thermometer (up to 0.3 degrees C), and the penetration depth (11.5cm or 6.0cm in one of the experiments) into the rectum (up to 0.4 degrees C difference between a penetration depth of 11.5cm and 6.0cm in one of the experiments). Differences in rectal temperature before and after defecation were minor (<0.1 degrees C). These results indicate that some care is required in generalizing rectal measures of body temperature.