1. Muscle fibres belonging to single motor units of identified type were studied in frozen sections of cat medial and lateral gastrocnemius muscles. Reconstruction of the distribution of fibres in individual units showed that the territories of all three physiological types present in the cat medial gastrocnemius were quite extensive. Within its territory, fibres belonging to the studied unit were distributed more or less uniformly without localized collections. The density of unit fibres suggests that, in cat medial gastrocnemius, a given region of the muscle may be shared by as many as fifty different muscle units.2. Direct determination of innervation ratios in identified muscle units required complete reconstruction of the three-dimensional distribution of unit fibres within the whole medial gastrocnemius. Satisfactory results were obtained with two type FF units and one type FR unit.3. Indirect estimates of the average innervation ratios expected for muscle units of different physiological type were obtained using counts of muscle fibres with characteristic histochemical profiles and data on relative frequencies of motor units of known type in the medial gastrocnemius unit pool. Such indirect estimates of innervation ratios agreed with the results of direct fibre counts in identified units for types FF and FR muscle units. Taken in sum, the available evidence suggests that an average muscle unit in the cat medial gastrocnemius contains between 400 and 800 muscle fibres, irrespective of physiological type.4. Tension production by single muscle units depends on a number of factors, including innervation ratio, the cross-sectional areas of unit fibres and the specific tension outputs of the unit fibres. The present results suggest that the specific tension output of gastrocnemius type S unit muscle fibres is considerably smaller (about 0.6 kg/cm(2)) than in either FF units (about 1.5-2.0 kg/cm(2)) or type FR units (2.6-2.9 kg/cm(2)).