We investigated the influence of ageing on the fatiguing characteristics of the mouse extensor digitorum longus (EDL) muscle as compared to those of the soleus muscle. Fatigue was produced by an intermittent stimulation protocol. We report for mature and aged animals the effects of fatigue on force produced during stimulation patterns that in non-fatigued muscle gave maximum force (Tmax, high frequency stimulation) and approximately half-maximum force (1/2Tmax, low frequency stimulation). In 15-month-old (mature) mice, fatiguing stimulation decreased Tmax in EDL and soleus muscle to 10.3 ± 1.0 % and 33.4 ± 3.0 % of control, respectively. In 30-month-old (aged) mice, the decrease in Tmax in EDL and soleus was statistically equal to that of the younger animals. Fatiguing stimulation decreased 1/2Tmax in EDL and soleus from 15-month-old animals to 22.5 ± 2.9 % and 45.7 ± 0.3 % of control, respectively. In 30-month-old animals, the 1/2Tmax in EDL and soleus muscle decreased to 18.2 ± 1.3 % and 35.0 ± 3.6 % of control, respectively. Under all conditions, the soleus fatigued significantly less. Contractile recovery from fatiguing stimulation was complete for the soleus in both age groups after 30 min, but incomplete for the EDL. The 1/2Tmax/Tmax ratio significantly increased in EDL and soleus muscle from 15-month-old animals after fatiguing stimulation. This increase was less significant in EDL, and absent in soleus muscle, from 30-month-old animals. These results indicate that fatiguing stimulation induces a leftward shift in the force-frequency relationship in the young animals; this shift is either significantly less (EDL) or absent (soleus) in the older animals. We speculate that the leftward shift of the force-frequency relationship may reflect a protective mechanism in younger animals against some of the damaging effects of fatiguing stimulation (i.e. oxidative stress). Experimental Physiology (2002) 87.1, 77-82.