A lesion limited to the dorsal columns, at the level of L3-L4, was carried out in chronic cats. This operation produced a "partial axotomy" type of lesion on the ascending branches of Ia hind limb afferents. Two to six months after this operation, intracellular studies on L7-S1 motoneurons were carried out. Similar studies were done in normal animals. The peak amplitude and the rate of rise (dV/dt) of heteronymous EPSP's were studied during control conditions (sampling at 1 Hz) and during the post-tetanic potentiation produced by a 500 Hz tetanus (for 3 s). The analysis of these synaptic potentials makes us conclude that: The amplitude of the enlarged EPSP's, observed during PTP, seems to be linearly dependent on their amplitude during control (i.e., pre-tetanus) conditions. Judging by their amplitude, there is no difference between potentiated EPSP's of operated and normal animals. There is also a linear relationship between the rate of rise of EPSP's and their peak amplitude. The slope of this relationship becomes steeper after "partial axotomy", i.e., for a given EPSP amplitude, the dV/dt of its rising part is steeper in operated cats. This steeper slope is also present in EPSP's studied during PTP. The sharper rate of rise of EPSP's, induced by the "partial axotomy" of Ia fibers, would be the mechanism behind the larger monosynaptic reflex previously observed in these operated cats.