1. The contraction kinetics of single striated muscle fibres and small fibre bundles from the frog and the toad were measured when the load was changed from P(0) to L < P(0). Simultaneous recordings were made of displacement at one end and force at the other end of the preparation.2. After the load was changed, the contractile force generally reached a steady value before the contraction velocity became steady. The amount of time required for isotonic contraction to become steady depended on the change in fractional load and on the temperature; it did not depend on sarcomere length in the range 2.2-3.0 mu or on the number of fibres in the preparation. The characteristics of the non-steady state are described in terms of the displacement deviation (the difference between the actual displacement at a given time and the back extrapolation of the steady phase of the displacement record) and the null times (the times at which the displacement deviation became zero, measured relative to the time at which the contractile force first reached the value of the load).3. The time average of the transient velocity was approximately equal to the final steady velocity.4. The product of the null time following a given relative force step and V(max), the steady velocity of unloaded contraction, was found to be independent of temperature. This is taken as evidence that the isotonic velocity transients originate in the contractile mechanism.5. The non-steady state following step changes in load is identified with the motion of cyclic contraction mechanisms. The motion of the specific model formulated by A. F. Huxley (1957) was compared with that of frog muscle fibres and, although the transients in the two systems differ in detail, the characteristic dimensions are of the same order.