The purpose of this paper is to provide insight into the alterations of 6-phosphofructo-1-kinase (PFK) activity and isozyme types of rat skeletal muscle during development and aging. PFK isozymes are tetramers which may be comprised of one or any combination of the three subunit types, L, M, and C. The effects of fusion or terminal differentiation of cultured rat L6 myoblasts leading to formation of myotubles does not have a noticeable effect on total PFK activity. However, the amount of M-type subunit was increased; and the level of the C-type subunit decreased. These subunit changes caused shifts in the isozymic types. The ultimate effects of prenatal development of PFK were characterized in the near-term fetal muscle. This stage of development was accompanied by a significant loss of the C-type subunit and by two-fold increases in the L-type and M-type subunits which accounted for the 40% increase in total PFK activity. After birth, the M-type subunit increased dramatically as did the total PFK activity. Since the L-type and C-type subunits were gradually lost during the subsequent 3 weeks, the homotetramer of the M-type subunit (M4) was the only type which is present in mature muscle. M4 persisted as the only detectable form in skeletal muscle during the remainder of life, but the total PFK activity and amount of M4 decreased after 18 months of age. The decreased total PFK activity in aged skeletal muscle suggested that the expression of PFK genes may have reverted to an immature state when total PFK activity was lower. As shown by both the immunological analysis and direct quantification of subunit types, this clearly did not occur. That is, the loss of PFK activity in aged muscle is a consequence of decreased levels of the M-type subunit and not reappearance of other subunit types such as found in maturing muscle. Further, our examination of aged skeletal muscle indicates that no significant structural changes in M-type subunits had occurred and that inactive or partially active proteins which could crossreact with the M-type subunit were not detectable. It is suggested that the loss of M4 could cause a depression of the glycolytic rate leading to diminished ability of senile muscle to accommodate extreme energy demands.