The time course of the contraction and the relaxation of individual contractions during incompletely fused tetani of motor units were analyzed. Investigations were performed on fast fatigable (FF), fast resistant (FR) and slow (S) motor units of the rat medial gastrocnemius muscle. Stimulation of a motor unit with a series of nine trains of stimuli at a frequency from 10 to 150 Hz was used and tetani fused to a variable degree were recorded. For fast motor units the procedure was repeated twice and observations were made on potentiated tetani in the second series of stimulation. For each tetanus, the amplitude of the tension increase, the peak amplitude of the contraction, the contraction time and the half-relaxation time were measured in the last contraction of the tension recording. It was observed in all three types of motor units that the last contraction was prolonged in parallel with the increase of fusion of a tetanus. In this contraction, the contraction time slightly decreased whereas the half-relaxation time strongly prolonged. The prolongation of the half-relaxation time was the strongest in tetani of slow units. Moreover, for the last contraction in a tetanus, the rate of changes in tension were studied. The rate of increase in tension during the contraction decreased in parallel with the increase of fusion of a tetanus, whereas the maximal rate of the tension decrease during the relaxation was found in tetani with fusion indices of 0.79, 0.98 and 0.95 for FF, FR and S motor units, respectively. Changes in the time course of contractions in tetani fused to a variable degree can shed light on processes of summation of contractions in unfused tetani at the level of individual motor units.