Tension changes have been measured during shortening or stretching movements applied to actively contracting motor units of the tonus bundle of the iliofibularis muscle of the toad Bufo marinus. During a slow, constant-velocity release tension fell, initially rapidly and then more slowly. The size of the fall, particularly later in the movement, depended on a number of factors including the duration of the isometric contraction before the onset of shortening, the amount of tension developed by the motor unit and the length of the muscle. When an isometrically contracting motor unit was rapidly shortened, the rate of rise on re-development of tension following the release was significantly slower than at the onset of the contraction. This effect was more marked if the release was preceded by a longer period of isometric contraction, if the experiment was carried out at shorter muscle lengths or if a smaller motor unit was used. If, following a period of isometric contraction, stimulation was interrupted, and a release-stretch movement applied to quickly bring the level of force down to near zero, and then stimulation recommenced , the final level of re-developed tension was less than that immediately before the release. The size of the tension deficit following re-development was larger for small motor units and at short muscle lengths. When the duration of the contraction before release was increased the size of the deficit also increased. A deficit could be prevented if the muscle was allowed to relax passively before the shortening movement was commenced. Stretch of actively contracting slow muscle produced an initial steep tension rise followed at times by a transient fall before tension slowly rose again. The transient fall became larger at short muscle lengths, and after long-duration contractions before stretch. Its tension dependence was less easy to establish because of complications involving changes in the relative series compliance. All of the above observations could be accounted for by an explanation based on the development of sarcomere non- uniformities in slow muscle fibres, produced as a result of non-uniform activation of the fibre membrane through the distributed nerve supply.