The sarcomere length of the rabbit pterygoid, temporal and digastric muscles was determined in seven mandibular positions. Six positions are representative of mastication, one of gnawing. Jaw displacement has a different effect in jaw openers (digastric and lateral pterygoid) and closers. The sarcomeres of the openers are long when the jaw is in molar occlusion and shorten relatively little at jaw opening. In the jaw closers they are very short in molar occlusion and become strongly extended when the mouth is open. Timing of electrical activity and sarcomere shortening corresponds closely. All muscles become active at sarcomere lengths of 2.6-3.1 micrometers. In the jaw closers, sarcomeres shorten till 2.1-2.3 micrometers during activity; in the jaw openers they do not shorten beyond 2.6-2.7 micrometers. Hence the openers remain close to optimum length (2.7 micrometers) while the closers shorten to lengths where possibly only 60% of the maximal isometric force can be exerted. During the initial phase of jaw closing, the medial pterygoid and temporalis shorten at speeds estimated to approach 50% of the presumed maximum speed. During the rest of the closing and power stroke, contraction takes place under near-isometric conditions, with little loss of force due to shortening. The jaw openers act at low shortening velocities.