Contractile, histochemical and biochemical properties of the triceps surae were compared in 13 aerobically trained male subjects aged 63 to 76 years. Electrical stimulation of the triceps surae was used to determine muscle twitch, tetanic, and fatigue parameters. From these tests, twitch tension (Pt), time to peak tension (TPT), half relaxation time (RT), tetanic tensions at 10(Po10), 20(Po20), and 50(Po50) Hz and a fatigue index (FI) were calculated. Muscle samples from the belly of the lateral head of the gastrocnemius were obtained using the needle biopsy technique. A portion of each sample was tested histochemically for myosin ATPase (pH 4.30, 4.58 and 10.00) and NADH-tetrazolium reductase in order to classify fibre types (ST, FTa, FTb) and to determine fibre areas. The remainder of each sample was analysed for succinic dehydrogenase (SDH) and phosphofructokinase (PFK) enzyme activities. Significant correlations were found between fibre areas (both ST and FT) and Po10/Po50 and FI. No significant relationships were noted between Pt, 1/2RT and MVC and any histochemical parameter. SDH and PFK activities did not correlate significantly with any histochemical or physiological parameter. Regular endurance exercise apparently does not retard the decline in contractile properties seen with the aging process. Substrate related regulatory enzyme activities (PFK) and marker enzyme activities (SDH) from skeletal muscle of the elderly are much lower than activities found in younger subjects, despite regular aerobic exercise.