Electronic transponders (3 x 18 mm) were injected into the base of the auricle of 69 10-day-old pigs, 111 four-week-old pigs and 24 six-month-old pigs, to examine the procedure of injection in the auricle, the tissue reaction and the ease of removal from the carcase. The injection was difficult in the 10-day-old pigs, but was easier at four weeks. One of the 204 transponders were lost and five of them appeared to be broken. Only one gilt showed some exudate around the transponder. The transponders could easily be removed when the pigs were slaughtered by cutting off the ear. The position of the transponder was significantly (P < 0.05) more ventral in the animals injected at 10 days old than in those injected at four weeks. The mean thickness of the connective tissue capsule around the transponders increased at first (P < 0.1) and then decreased when they were injected into 10-day-old pigs but decreased (P < 0.1) from shortly after injection in the pigs injected at four weeks old. After five months, the capsule in the animals injected at 10 days old was on average significantly (P < 0.01) thinner than in those injected at four weeks.