1. Comparisons were made in vitro at 25 degrees C among soleus and extensor digitorum longus (EDL) muscles from young (2-3 months), adult (9-10 months), and aged (26-27 months) male mice. We tested the hypotheses that, compared with soleus and EDL muscles of young and adult mice, those from aged mice develop decreased maximum tetanic force (P0, mN) and specific P0 (N/cm2), and that no significant differences occur for contraction time, half-relaxation time, or force-velocity relationship. 2. For the aged mice, the P0 of the soleus muscles and EDL muscles were 78 and 73% respectively of the values for adult mice. The specific P0 of EDL muscles of aged mice was 78% of the value of 23 N/cm2 obtained for young and adult mice. For soleus muscles, the specific P0 of 21 N/cm2 did not change with age. 3. Compared to values for young and adult mice, the contraction and half-relaxation times of soleus muscles from aged mice were increased, but the overall force-velocity relationships of soleus and EDL muscles did not change. The pooled values for the maximum velocity of unloaded shortening extrapolated from the force-velocity relationship of soleus and EDL muscles were 4.6 and 10.1 fibre lengths/s, respectively. 4. The decrease in the specific P0 of the EDL muscle with ageing must result from either a decrease in the number of cross-bridges in the driving stroke or a decrease in the force developed by each cross-bridge.