The rate of metabolic kinetics and the frequency of biological cycles may be correlated with the rate of aging and the maximum life-span potential. Therefore, investigations either into changes with age of such parameters within one species or into differences between species may give some information about the genetic programming of the aging process. Male Sprague-Dawley rats aged 3.5, 7, 12, 17, 23 and 33 months (m) were used to determine the changes with age of those metabolic pathways mentioned in the title, using the liver, kidney, brain, heart and the skeletal muscle. The maximum percentage of glucose utilization via the pentose pathway, compared to the total glucose utilization, was calculated after intravenous administration of D-[1-14C]- and D-[6-14C]glucose by the determination of the trioses (as lipids) 3 hours after the application. Glycogen kinetics was determined analogously. Total protein metabolism was observed using the essential amino acid L-[2,5-3H]histidine. The results indicate a decrease in the glucose utilization via the pentose pathway in the course of aging in liver, kidney, heart and skeletal muscle and a decrease from 3.5 months on in brain, a small but not significant change of the kinetics of glycogen metabolism (a lower turnover), and a reduced rate of protein synthesis in liver, kidney, heart and brain through an age of 23 months, followed by an elevated rate. Brain did not show any changes. The reduction of the pentose pathway may possibly be the cause of higher lipofuscin accumulation in the cells of some organs, lacking sufficient reduction equivalents for lipid metabolism. Furthermore, there could exist a connection with the reduced protein turnover, because less riboses are provided for the synthesis of nucleic acids.