SummaryThe basal adenylate cyclase activities, per g wet wt, of homogenates of adult skeletal muscle (4 ± 0.4 S.E. nmoles/10 min · g wet wt−1) were lower than those values in 150‐ and 80‐day fetal muscle (18 ± 1.2 and 23 ± 1.6 S.E. nmoles/10 min · g wet wt−1). This difference in enzyme activity relative to growth was also apparent in the 100,000 ×gparticulate fractions of adult muscle (2.5 ± 0.2 S.E. nmol/10 min·g wet wt−1) compared to the 150‐and 80‐day fetal muscle (12 ± 1.5 and 14 ± 1.9 S.E. nmoles/10 min·g wet wt−1). The fluoride‐stimulated adenylate cyclase activities of the homogenates and particulates and the increases in enzyme activity with fluoride were also at least four‐fold greater in 80‐ and 150‐day fetal than in adult muscle. When basal adenylate cyclase activities of homogenates were compared on the basis of the nitrogen content of the fraction analyzed rather than on the wet weight of the original samples, the differences in enzyme activities of the adult compared to the fetal series were even greater because of the lower nitrogen content of fetal muscle (1.8 ± 0.2 S.E. nmoles/10 min·10 mg N−1for the adult series and 10.2 ± 0.7 and 22.2 ± 1.2 S.E. nmoles/10 min·10 mg N−1for the 150‐and 80‐day series). The difference in activity with age was again apparent in the 100,000 ×gparticulate fractions (1.9 ± 0.2 S.E. nmoles/10 min·g wet wt−1for the adult series and 9.8 ± 0.9 S.E. and 24 ± 2.2 nmoles/10 min·g wet wt−1for the 150‐ and 80‐day series). Because the nitrogen content of the 80‐day fetal muscle was less than that of the 150‐day series, the adenylate cyclase activities per mg N were highest in the younger fetal series. In the presence of guanylylimidodiphosphate the percent increase in adenylate cyclase activity was two to three times greater in the fetal particulate preparations than in the adult and was concentration‐dependent in both series.The basal guanylate cyclase activities for both the 100,000 ×gsupernatant and particulate fractions were higher in 80‐day fetal than in adult muscle; two‐ to three‐fold higher in terms of wet wt and over six‐fold higher in terms of nitrogen. Both the absolute and percentage increases in activity of the fetal particulate enzyme with Triton were at least twice those of the adult. Triton had no effect on the enzyme activity of the 100,000 ×gsupernatant fraction of either series. The concentration of cyclic adenosine 3′: 5′‐monophosphate was 8 to 10 times higher in rapidly growing than in adult muscle, and the concentration of cyclic guanosine 5′‐monophosphate was 26 to 62 times greater.SpeculationThere is a striking difference in the degree of maturation at birth of skeletal muscle in different species of animals. Skeletalmuscle from the rhesus fetus is particularly suitable for study if one is interested in human fetal muscle metabolism because the maturation rate of this tissue (on the basis of percent of gestation) is similar in the two species. Because fetal tissues must proliferate and differentiate, they must perform synthetic processes at a rapid rate, and synthetic processes are expensive in terms of energy. Therefore, it is not surprising that a number of fetal enzymes are more active than are those of the adult. In addition, a greater sensitivity of fetal enzymes to effector molecules may further magnify the activity of these enzymes. It would be of interest to measure the cyclic nucleotide‐dependent protein kinase activities in fetal and adult rhesus muscle. Recent evidence suggests possible differences in the functions of types I and II cAMP‐dependent protein kinases in relation to cellular growth and differentiation. Studies of these kinases in rhesus fetal muscle during the predominantly proliferative phase (earlier than 85 days gestation) and during the period when differentiation is predominant (85 to 110 days) should be of value in studying the biologic significance of these kinases.