The presence of parvalbumin, a calcium-binding protein, has been correlated with the maturation of locomotor activity in developing striated muscle. In the present study, postnatal parvalbumin immunoreactivity is examined in the tibialis anterior, intercostal, diaphragm and intrinsic muscles of the tongue of the rat to gather a better understanding of the different developmental patterns. Parvalbumin immunoreactivity appears in the anterior tibialis muscle by day 4, and reaches an adult checkerboard pattern 2 days later. In contrast, parvalbumin immunoreactivity in the intrinsic muscles of the tongue, and in diaphragm and intercostal muscles, which are active near birth, does not appear until the 2nd week. Therefore, these features suggest that parvalbumin immunoreaction is not exclusively dependent on functional activity. In addition, the finding that differences in parvalbumin expression do not correlate in time with the differentiation of fiber types as judged by myosin ATPase activity, suggests that myosin and parvalbumin might be regulated by different mechanisms.