Abstract The profile of excitatory postsynaptic potential (EPSP) produced on triceps surae motoneurons by 5 msec duration triangular wave stretch of de-efferented medial gastrocnemius (MG) and soleus (S) muscles was observed on anesthetized, spinalized cats. With the muscle held at light initial tension a stretch of 1–5 μm amplitude produced a ‘threshold’ EPSP of 25–100 μV. This amplitude is within the range of that reported for synaptic excitation by a single Ia fiber. With the muscle at moderate initial tension, a 5 μm pull on the homonymous muscle produced a well-defined EPSP of 0.4–0.8 mV. As stretch was increased to 60 μm the EPSP grew progressively to a peak of 2–3 mV. Additional stretch to 200 μm had little effect on EPSP size. A second EPSP 1.3–1.5 msec after the first developed with a stretch of 45–60 μm. At 200 μm stretch its size was 50–100% of the first EPSP. Parallel changes occurred in the profile of afferent nerve volleys recorded at the dorsal root entry zone. These data, together with our previous information on isolated afferents, suggest that both volleys are of Ia origin. Stretch-evoked EPSPs were slightly longer in rise time but similar in size and half-decay time to EPSPs evoked by electrical stimulation of the muscle nerve. These results demonstrate the feasibility of determining the ‘strength’ of synaptic connection between Ia afferents and motoneurons by use of a mechanically activated volley of pure Ia origin.