This research investigated the performance and placement of an injectable radio frequency identification (RFID) and temperature sensor to monitor the body temperature of horses. Eleven sensors were calibrated to assess reliability (accuracy and repeatability) of the temperature readings. Results of four separate calibration trials demonstrated significant variability in both accuracy and repeatability. To quantify accuracy, the regression standard errors (SE) were placed into four performance categories: 4, 0, 2 and 3 excellent (SE 〈 0.5°C); 2, l, 7, and l good (0.5°C < SE 〈 0.75®C); 2, 3, 0, and 7 marginal (0.75°C < SE 〈 1.0°C); and 3, 7, 2, 0 poor (SE < l.0°C) sensors in each of the performance categories for the four calibration trials, respectively. Three of the eleven sensors evaluated were found to be repeatable, however, with marginal accuracy. Based on these results it is recommended that the temperature system be calibrated before use.