Tail autotomy in Proechimys cuvieri was studied both morphologically and histologically. The rupture always occurs at the base of the external tail, e.g. in the immediate vicinity of its junction with the body. It thus concerns the whole caudal appendage. The distal epiphysis is separated from the fifth caudal vertebra and lost with the rest of the tail. There is no single reason responsible for the constancy of this breaking point, but several morphological factors can act together: these include strong binding of the five first caudal vertebrae to the body, disappearance of the plurisegmental muscles beyond this level, and the great extent and loose structure of the epiphyseal plates. Autotomy is a biological event occurring throughout the life of the animals, but it is of a cumulative nature. Tail loss is much more prevalent in older and heavier animals than in juveniles. Overall, about 9% of wild populations show this loss. Owing to the increasing percentage of occurrence from young to old, tail autotomy seems to enhance the survival chances of its owner, although direct proof of any predation influence are still lacking.