Muscle fibre composition, fibre areas and enzyme activities were determined in muscle biopsy samples (m. vastus lateralis) from both legs of 34 clinically healthy men and 31 clinically healthy women (aged 20-70 years). No significant difference was found in any of the three variables between the right and left leg. In the whole material the mean percentage of type I fibres was significantly higher in men, whereas that of type IIA fibres was significantly higher in women (men 58% I, 27% IIA, 13% IIB, women 51% I, 32% IIA, 15% IIB). The areas of all fibre types were significantly larger in men than in women. When the subjects were divided into age groups of 20, 30, 40, 50, 60 and 70 years, respectively, the fibre composition was found to be similar at different ages in both men and women, but changes in fibre areas were observed. The 70-year-old men and women showed significantly reduced areas of both type I and II fibres compared with the 60-year-olds. Only minor changes were seen in enzyme activities in relation to age. Men of the youngest age group had significantly higher levels of citrate synthase activity than those of the older age groups. Similar findings were seen for women except for the 60-year-olds, who had as high activity levels of citrate synthase as the 20-year-olds.