The individual EPSPs evoked by the action of single Ia fibers from cat triceps surae (MG, LG, SOL) were recorded in homonymous and heteronymous motoneurons innervating these same three muscles. 2. In general, Ia fibers projected to a greater percentage of homonymous than heteronymous motoneurons. One class of Ia afferent evoked EPSPs in virtually all homonymous motoneurons; the other had a substantially lower projection frequency. Possible difficulties introduced by the limited resolution of the averaging technique are discussed. 3. Individual EPSPs were larger on the average if evoked a) in SOL rather than in MG or LG motoneurons, b) by LG rather than by MG or SOL afferent fibers, or c) in homonymous rather than in heteronymous motoneurons. The mean EPSP was larger in homonymous than in heteronymous motoneurons because the largest EPSPs (greater than 150 muV) were found mainly in homonymous motoneurons. 4. Rise times of EPSPs were only slightly shorter in homonymous than in heteronymous motoneurons, suggesting that other factors besides relative location of Ia terminals account for the observed EPSP amplitude differences. Rise times in SOL motoneurons were longer than those in MG or LG. 5. LG afferent fibers tended to produce larger EPSPs in rostral than in caudal LG motoneurons, and MG afferents produced larger EPSPs in caudal than in rostral MG motoneurons. These spatial effects were in accord with the more rostral entry of LG than MG Ia afferents into the spinal cord. The differential projection of SOL afferents to MG and SOL motoneurons which overlap spatially in the spinal cord suggests a species specificity in addition to a location specificity.
