Unilateral section of dorsal roots C5 to T1 were performed in rats, and the automutilation behavior was measured by the extent of the limb lesions expressed in arbitrary units. Changes in the scores of automutilation were studied after the injection of four substances: a morphinomimetic (pethidine) and three antibiotics, chloramphenicol, amoxicillin, and doxycycline. Effects were tested on groups of at least eight rats that were compared with another group of eight animals operated on in the same way but injected with distilled water. The animals of the group treated with pethidine performed significantly less autotomy than did the animals in the control group. The same effect was found when the animals were injected with chloramphenicol and amoxicillin. On the contrary, doxycycline was found less efficacious. These results are discussed using the hypothesis of autotomy caused by an abnormal painful sensation felt in the deafferented forelimb.