1. Firing patterns of Ia afferents and motoneuron efferents were recorded while subjecting homonymous de-efferented triceps surae muscle to vibration applied longitudinally to the tendon.2. The amplitude of vibration (ΔL) necessary to elicit the preferred firing rate of each motoneuron was noted over a wide range (20 to 180cps) of vibration frequencies (f). This permitted the plotting of a ΔL-f curve for each motoneuron, the area above the curve being termed the “response area” and the frequency associated with the minimal ΔL being termed the “optimal frequency” of vibration. In general preferred firing rates ranged from 6 to 13imp/sec and the optimal frequency was near 100cps.3. These parameters of vibrator responsiveness were measured before and after post-tetanic potentiation. They were also measured before and during concomitant stretch of a synergist and stimulation of antagonistic afferents. The facilitatory effect of post-tetanic potentiation and stretch of the synergistic muscle resulted in a broadening and downward shift of the Δ-f curve to the right. This indicated an increase in response area and shift in optimal frequency to a higher value. Reverse inhibitory effects were noted during stimulation of antagonist afferents.4. Similar parameters were generated for Ia afferents by studying their vibrator responsiveness at arbitrarily fixed rates of 20 and 10imp/sec. The optimal frequency of vibration was quite similar for afferents (20imp/sec) and efferent, as were the ΔL values associated with this frequency. The further vibration frequency was removed from the optimal frequency the more pronounced became the difference in ΔL requirements, far less amplitude of stretch being necessary to maintain the selected Ia firing rate. There is a discussion of mechanisms underlying the difference in profile of ΔL-f curves for Ia afferents and motoneurons.