A frog ventricular muscle strip could be fully tetanized by alternating current stimulation at 10 Hz and 20 V/cm in a solution containing 9 mM Ca2+. During isometric tetanus, the controlled release was made and the shortening velocities against various loads were measured. The isometric force was varied by reducing the stimulus intensity in K+-rich solution, or by reducing the external Ca2+ concentration. The force-load-velocity relation was described by a simple hyperbolic equation: (P+A)(v+b)=b(F+A), A=(F/Fm)a for shortening, and (2F-P+A')(-v+b')=b'(F+A'),A'=(F/Fm)a' for lengthening, where F is the isometric force, Fm is the maximum isometric force at the optimal muscle length, Lm, P is the load, v is the velocity, a, b, a' and b' are constants. The values of constants were a/Fm=0.51, b=0.75 Lm/sec for shortening and a'/Fm=0.39, b'=0.75 Lm/sec for lengthening at 20 degrees C. At muscle lengths shorter than 0.92 Lm, the internal load defined as the difference between the external load and calculated load at a given velocity increased in proportion to both the velocity and the decrease in muscle length.