Human gastrocnemius and slow twitch muscles contain phosphodiesters that may be detected in vivo by phosphorus nuclear magnetic resonance (NMR). This work represents a study of 354 spectra obtained from healthy subjects of various ages and from patients with peripheral vascular disease. The analysis of the data indicate a correlation between the concentration of phosphodiesters and age. By comparing the data obtained with healthy subjects and patients it is concluded that the increase in phosphodiesters is not due to disease, but to ageing itself. The significance of this increase is discussed.