1. Steady-state force-pCa relations were determined for mechanically skinned fibres of fast- and slow-twitch rat skeletal muscle, extensor digitorum longus (e.d.l.) and soleus respectively, at varied sarcomere lengths and temperatures (3-35 degrees C).2. This relation was different for the two fibre types at all sarcomere lengths, with soleus fibres having lower Ca(2+) concentration threshold for activation and requiring a wider Ca(2+) concentration range to reach force saturation.3. An increase in sarcomere length within the range 2.2-3.6 mum reversibly shifted the steady-state force-pCa curves for both fast- and slow-twitch fibres toward lower Ca(2+) without affecting the steepness of the curves.4. The results are consistent with the idea that the sensitivity of the contractile apparatus to Ca(2+) is increased with increasing sarcomere length.5. The change in apparent Ca(2+) sensitivity (measured as the change in pCa corresponding to 50% Ca-activated force (DeltapCa(50))) was approximately proportional to the change in sarcomere length (DeltaSL) at all temperatures for both types of muscle fibre.6. The proportionality coefficient (DeltapCa(50)/DeltaSL) was higher for slow-twitch fibres than for fast-twitch fibres by a factor of 1.5-2.0 at all temperatures investigated.7. The force-pCa relation was less affected by changes in sarcomere length at lower (3-5 degrees C) than at higher temperatures (22-35 degrees C).8. The sarcomere length-maximum force relation obtained with skinned fibres for sarcomere lengths between 2.10 and 4.05 mum was similar for both fibre types at all temperatures despite large temperature dependent variations in maximum Ca-activated force response. Maximum force decreased linearly to zero between ca. 2.8 and 3.95 mum; remained high and essentially constant between ca. 2.40 and 2.80 mum, and decreased significantly below 2.40 mum.9. Myofilament-generated tension oscillations produced by partially activated soleus skinned fibres are highly length dependent, disappearing completely when sarcomere length is increased above 3.15 mum.