1. Gender differences in fat oxidation at rest and during exercise may contribute to higher body fat in women. We examined gender differences in fat oxidation at rest and during submaximal exercise and their relationship to sympathetic nervous system activity, free fatty acid availability, body composition and aerobic capacity in older volunteers. 2. We measured free fatty acid kinetics using [14C]palmitate, absolute (micromol/min) and relative (respiratory quotient) rates of fat oxidation by indirect calorimetry and sympathetic nervous system activity from noradrenaline kinetics using [3H]noradrenaline in 12 older men (70+/-4 years) and 12 older women (66+/-4 years) at rest and during 30 min of submaximal exercise (45% of peak oxygen consumption). 3. At rest, men oxidized more fat than women on both an absolute (88+/-19 versus 51+/-15 micromol/min; P<0.01) and relative (respiratory quotient: 0.80+/-0. 04 versus 0.85+/-0.04; P<0.01) basis. These differences were not related to noradrenaline appearance rate, free fatty acid concentration, body composition or aerobic capacity. During exercise, fat oxidation was higher (P<0.05 to P<0.01) in men on an absolute level, but respiratory quotient did not differ. Higher absolute fat oxidation in men during exercise was explained by their higher absolute workload. Plasma free fatty acids and free fatty acid rate of appearance did not differ between men and women during exercise despite higher (P<0.05 to P<0.01) plasma noradrenaline concentrations in men. 4. We conclude that: (i) resting fat oxidation is higher in older men compared with older women independent of differences in noradrenaline appearance rate, free fatty acid availability, body composition or aerobic capacity, and (ii) despite higher plasma noradrenaline concentrations during submaximal exercise, no gender differences in free fatty acid appearance rate or fat oxidation were found. These results suggest a sex dimorphism in post-absorptive fat metabolism in the elderly.