This study investigated an injectable radio frequency identification (RFID) and temperature sensor (TX1400 B, Digital Angel, Inc., St. Paul, MN-USA). The sensor is implanted into the neck and used to measure temperature and a unique identity code. Eleven new sensors were calibrated to assess reliability. Results of four calibration trials demonstrated significant variability in both accuracy and repeatability. To quantify accuracy, the regression standard errors (SE) were placed into three categories: There were 4, 0, 2 and 3 Good (SE ≤ 0.5°C); 4, 4, 7, and 8 Marginal (0.5°C < SE ≤ 1.0°C); and 3, 7, 2, and 0 Poor (SE > 1.0°C), respectively. Three of the eleven sensors evaluated were found to be repeatable, however with Marginal accuracy. Regression results for each sensor are provided. Based on these results the system was determined to be unreliable for this application. Three necropsies were performed on horses with previous illness. The purpose of the necropsies was to learn more about the anatomy of the horse's neck, to locate previous implants using an ultrasound, and find the ideal location for the RFID temperature sensor. Most sensors were located in the muscle tissue of the neck. The ideal location was found to the fat section of the neck under the mane.