Ageing is associated with a reduction in muscle carnosine (β-alanyl-L-histidine), but there are no data on the changes specifically in type I and type II muscle fibres. Given the higher carnosine content of type II fibers, changes observed in whole muscle may be secondary to a shift in fibre composition. Carnosine, β-alanine, histidine, taurine, and citrate synthase (CS) and glycogen phosphorylase (Phos), were measured in pools of single muscle fibres from freeze-dried muscle biopsies of vastus lateralis of nine elderly sedentary subjects (65-80 years) with osteoarthritis of the knee and undergoing total knee replacement, and nine young moderately active healthy subjects (20-35 years). Fibres were characterised as type I or II by myosin ATPase activity. Carnosine was 53.2% lower in type II fibres of older subjects resulting in an estimated 7% (and most probably still higher) decline in intracellular physico-chemical buffering capacity. Younger subjects showed higher CS activities in type I and higher Phos activities in type II fibres. These differences were less apparent in elderly subjects. Possible causes for the change in the carnosine content are reduced physical activity, reduced meat intake, or the result of progressive denervation.