SUMMARY1. Peak twitch force (Fo) and sarcomere length (SL) were measured in trabeculae that had been dissected from the right ventricle of rat heart and that were superfused with a modified Krebs-Henseleit solution at 25 'C. Sarcomere length was measured by laser diffraction techniques. Force was measured with a silicone strain gauge. Unloaded velocity of sarcomere shortening (V0) was measured by the 'isovelocity release' technique.2. At [Ca2+]0 = 1P5 mm and SL below 1'9 #um, V0 increased in proportion to SL, while Vo was independent of SL above 1P9 jsm. At [Ca2+]0 = 0-5 mm, V0 was proportional to SL up to 2-2 /tm. At [Ca2+]0 = 0-2 mm, V0 was proportional to SL up to 2-3 jtm which is the longest SL that we were able to study in our trabeculae.3. A unique relationship was observed between V0 and F0, irrespective of whether F0 was altered by variation of [Ca2+]. or sarcomere length above slack length. Calculated stiffness of the elastic term of the viscoelastic element was independent of v, i.e. 45-50 N mm3.6. Fv was slightly larger in muscles that were superfused with Ca2+-free Krebs-Henseleit solution: at SL = 2-0-2-1 jtm, F, increased by 0-4 %F0 ,sm-s-i at[Ca2+]0 = 15 mm; at SL = 2X0-2-1 jtm in Ca2+-free solution, F, increased by 0 53 %F0 j#m-s-' at v < 5 ,tm s-1.7. Dynamic stiffness (DS) was measured as the force response to sinusoidal MS 9089 P. P. DE TOMBE AND H. E. D. J. TEB KEURS sarcomere length perturbations (500 Hz; SL = 11 +0'7 nm peak to peak). Dynamic stiffness during shortening was proportional to the load (L/FO) and was fitted to: DS= 1210+ 01084L/Fo (r = 0-84). The phase shift (() on the other hand, was independent of L: (F = 51P8+0105L (r = 001).8. The force-velocity relation calculated for one crossbridge on the basis of the observed relationship between DS and L/FO was close to linear, as is predicted by Huxley's (1957) model. 9. These results are discussed in relation to a model in which the unloaded shortening velocity of the cardiac sarcomere is limited by the passive viscosity of the muscle. The model adequately predicts the observed relation between V0 and Fo.