1. The tension and sarcomere length responses induced by ramp stretches (amplitude 1-3% of initial fibre length (Lzero) and speeds of 0.01-12 Lzero s-1) were examined, at 10 degrees C and sarcomere lengths of approximately 2.7 microns, in resting intact muscle fibre bundles isolated from the soleus (a slow muscle) and extensor digitorum longus (a fast muscle) of the rat. 2. In both fibre types, the tension response to moderately fast ramp stretches consists of a viscous, a viscoelastic and an elastic component. At low stretch velocities, where the viscous component is very small, the tension response consists of only the viscoelastic and elastic components. 3. The viscosity coefficient (mean +/- S.E.M., 2 +/- 0.01 kN s m-2, n = 12) and the relaxation time of the viscoelasticity (44 +/- 2 ms, n = 12) of the slow muscle fibres were significantly larger than those of the fast muscle fibres (0.8 +/- 0.1 kN s m-2 and 11 +/- 1 ms, respectively, n = 20). 4. The relaxation time, in either fibre type, is too long for the viscoelasticity to be due to rapidly cycling, weakly attached cross-bridges. Moreover, the tension components increased with sarcomere length and were insensitive to 5-10 mM 2,3-butanedione 2-monoxime (BDM), which inhibited active contractions. 5. The possibility that the fast-slow fibre differences may reflect differences in myoplasmic viscosity and connectin (titin) isoforms (in their gap filaments) is discussed.