The growth of the masseter muscle in eight infant, juvenile, and adolescent 
female rhesus monkeys (M. mulatta) was examined over a 2.5 year period using 
serial radiographic cephalometric techniques with the aid of radiopaque muscle 
markers. The radiopaque markers, which are composed of small pieces of root 
canal broach inserted into the muscle belly, make it possible to determine 
longitudinal masseter muscle growth as well as migration of the masseter muscle 
relative to the mandible. It was found that the masseter muscle increased in 
length by 64% during the total growth period, most of which occurred between 6 
and 18 months of age. Relative to the cranium, the masseter muscle grew markedly 
inferiorly and only slightly posteriorly. Relative to the mandible, the masseter 
migrated in a posterior and slightly superior direction, keeping pace with the 
ramus and condyle as they grew posteriorly and posterosuperiorly throughout the 
study period. It was concluded that: 1) radiopaque muscle markers are a valuable 
tool for analysis of muscle growth and alteration of muscle location; 2) the 
masseter muscle in the rhesus monkey undergoes elongation, probably due to 
addition of sarcomeres at the fiber-tendon junctions; and 3) posterior migration 
of the masseter muscle relative to the corpus of the mandible, probably due to 
the nature of its periosteal attachment, results in a stability of the 
anteroposterior position of the masseter muscle despite the anterior 
displacement of the mandible.