In cultured human skin fibroblasts the glucose flux through the hexose monophosphate shunt (HMS) amounts to 4% of the glucose flux through the glycolytic pathway. Upon in vitro ageing the rate of glucose utilization through the HMS is decreased more than 50%. This decrease in HMS was not caused by a limiting enzymatic capacity since glucose utilization through the HMS could be raised at least 30-fold in both 'young' and 'aged' fibroblasts upon stimulation with phenazine methosulphate. This effect of in vitro ageing upon glucose metabolism was also not due to differences in proliferation rate between 'young' and 'aged' human fibroblasts, since there was no difference in glucose utilization between proliferating and growth-inhibited (confluently cultured) fibroblasts. The NADPH/NADP ratio was found to be decreased by 12% in 'aged' cells.