The active length-tension curves of identified single motor units (MUs) belonging to peroneus longus muscle (PL) of anaesthetized adult cats were obtained by eliciting isometric single twitches and tetani. The recorded responses were evaluated by measuring the peak tension amplitude and the tension-time area at muscle lengths extending throughout the physiological length range of the muscle (mean 5.5 mm, standard deviation +/- 0.8). The muscle lengths at which each tested MU developed its maximal twitch (Ltw) and tetanic (Lte) tensions were determined and compared with the muscle length (Lo) at which the stimulation of all the alpha-axons, innervating PL and contained in L7 ventral root, developed their maximal twitch tension. The mean of single MU Ltw values was at Lo +1.08 +/- 1.1 mm. Slow MUs showed the longest values of Ltw (Lo +1.6 +/- 1.0 mm). Single MUs stimulated at tetanic frequencies presented their Lte at values shorter than Lo (Lo - 2.8 +/- 1.7 mm). Slow MUs had the shortest Lte (Lo - 3.4 +/- 1.5 mm). For all the units Lte was shorter than Ltw. Ltw and Lte were, respectively, negatively and positively correlated with the developed tension. Optimal length values also appeared to be related to the MU types. The possibility is discussed that the muscle and tendon compliances and the high non-linearities to the applied forces are the main factors which can determine the differences among Lo, Ltw and Lte values. The relationships between MU type and optimal length values are suggested to be, at least partly, an epiphenomenon due to the different contraction strengths of the various MU types.(ABSTRACT TRUNCATED AT 250 WORDS)