Afferent discharges were recorded from stretch receptors identified as Golgi tendon organs, in the medial gastrocnemius and soleus muscles of the cat. 2. The response of a tendon organ was recorded during stimulation of one or more motor units selected for the intensity of discharge elicited from the receptor during twitch and tetanic contractions. 3. Repetitive stimulation of a single motor unit could evoke in a tendon organ a firing rate of up to 174 impulses/sec. The mean rate for a total of 90 motor units was 65 (+/- 32 S.D.) impulses/sec. No significant difference in effectiveness could be detected between motor units covering a wide range of contraction speeds, tetanic tensions and susceptibility to fatigue. 4. The response of a tendon organ to contraction of several motor units in combination was greater than from stimulating any one motor unit alone but less than predicted from the algebraic sum of individual responses. 5. The relation between firing rate and tension was plotted for combined stimulation of up to ten motor units. The relation was found to be a straight line provided the size of the response elicited by each motor unit in the stimulated bundle was similar or when responses were ranked according to their intensity. When one motor unit evoked a much more powerful response than others it tended to dominate the discharge and disturb the linearity. 6. Evidence is provided that the sites of stimulus transduction for motor units which exert their effect directly on the receptor can be relatively independent of one another. It is argued that on such occasions summation of responses may be attributed to mechanisms operating at the level of impulse generation.
