The progressive transition from developmental to adult myosin isoforms during perinatal development was quantified in four muscles (diaphragm, gastrocnemius medialis, masseter and tongue) of four mammals (guinea-pig, hamster, rabbit and rat). It was observed that the timing of transition varied for each muscle, and differed according to the mammal as well. This suggests that the synthesis of adult myosin isoforms may be partly related to the specialized contractile function of a given muscle in a given species.