1. The contractile force was compared in isolated soleus muscles from young (2.5-8 months old) and aged (28-31 months old) mice. Force was measured at 25 degrees C during isometric tetanic contractions during isovelocity stretching and shortening contractions. 2. The normalized isometric force was lower by 13.3% in muscles from aged mice. Muscles from young and aged mice produced 0.951 +/- 0.031 N mg-1 (n = 12) and 0.824 +/- 0.048 N mg-1 (n = 9) respectively. The relaxation time, from 90 to 10% of the tetanic force, of muscles from aged mice was 102.1 +/- 3.7 ms (n = 6), which was longer than that for muscles from young mice, 84.4 +/- 3.8 ms (n = 6) (means +/- S.E.M.). 3. The force during shortening was also reduced in muscles from aged animals by the same proportion as the isometric force. Therefore the force during shortening relative to the isometric force was the same in muscles from young and aged mice. 4. During rapid stretching soleus muscles from aged mice produced a similar force to those from young mice. Therefore stretch can remove the weakness in muscles of aged mice. 5. These changes in muscles from aged mice are similar to those produced when inorganic phosphate (Pi) levels are raised, in skinned rabbit psoas fibres, or during fatigue or with low intracellular pH (pHi), in frog muscle. It is possible therefore that the force loss due to ageing may be due to a higher Pi level or a lower pHi.