Twenty-three male juvenile rhesus monkeys (Macaca mulatta) were used in an experimental study of long-term mandibular adaptations to induced protrusive function. Serial protrusive appliances were placed in 11 experimental animals and mandibular adaptations were monitored cephalometrically. Twelve animals were used as controls. After 48 weeks significant increases in increments of condylar growth and in overall mandibular length were noted in the treated animals. At the end of the 144-week experimental period, the mandibles of the treated animals were 5 to 6 mm longer than those of the control animals. The results of this study do not support the hypothesis that the mandible has a genetically predetermined length.