Primary endings were classified into three types by vibration threshold (1976) and a spike of motoneuron was elicited by acceleration sensitive Ia impulses (1993). The reports suggested that motoneuron is activated by Ia on which different types of intrafusal fibers are selectively induced by different vibratory stretch phases. Four cats were anesthetized with an initial intraperitoneal dose of 10% urethane and 1% chloralose (4cc/kg). Subsequent doses, as required, were given intravenously via a catheterized cubital vein. 10cc/kg of 10% urethane was given intravenously at the end of experiment. Left triceps surae muscle of cats were dissected free from surrounding tissues, while blood supply was kept intact. All other nerves in the left hip and leg were cut except nerves of the muscle. A cat was fixed on a frame mounted securely on an animal tablet and the calcaneus was attached firmly by a metal. The dorsal and ventral roots between L6 and S1 were exposed by the conventional 1aminectomy. Thin afferent and efferent filaments were hooked on each pair of electrodes at about 20mm from their entry into the spinal cord and motoneuron activity was recorded by antidromic electric stimulations of motor nerve. An afferent fiber (over 80m/s) was identified as a spindle ending Ia by their behavior during twitch contraction of the muscle elicited by stimulating its nerves. Impulses of single Ia and intracellular neuron activities were recorded simultaneously while tendon of medial or lateral gastrocnemius muscle or soleus muscle was stretched by 100Hz vibration of amplitude 100μm for 0.5 sec every 2 sec and vibration was repeated 5 times. The muscle was contracted by electric single shock stimulations before stretches. Ia impulses were classified into three types corresponding to a sensitive cyclic phase of sinusoidal stretch at which Ia impulses were elicited. Impulse of acceleration sensitive Ia was discharged at initial phase of vibratory stretch and elicited a motoneuron spike. Velocity sensitive and displacement sensitive Ia impulses were driven while the muscle was stretched in 37 cycles. Latency of velocity sensitive Ia impulses from the vibratory stretches was 4.26±0.05ms and latency of displacement sensitive Ia was 4.92±0.05ms in 37 cycles. All of EPSPs and initial serial 4 spikes of motoneuron were driven by Ia impulses which was induced by 100Hz vibration. But spikes of motoneuron appeared intermittently every 2 or 3 Ia impulses except initial 50ms during 500ms vibration. It is suggested that velocity sensitive Ia impulses elicit motoneuron spikes rapidly in early period when muscle is stretched but motoneuron activity is inhibited gradually by some reflexive inhibitory mechanism as stretches are repeated although physical property of primary endings is not changed during vibration.