The ability of voltage-dependent inward currents (likely Na(+)) of the adult cat lumbar motoneurons to amplify rapidly changing (i.e., dynamic) synaptic inputs was investigated using in vivo intracellular recording techniques. Fast amplification was assessed by measuring the magnitude of the high-frequency (180 Hz) component of the Ia synaptic input due to tendon vibration as a function of somatic voltage and was compared with the previously observed amplification of steady inputs (steady state response of PICs to slow inputs). Data from 17 experiments show that amplification of the dynamic input indeed occurred and was directly linked to neuromodulatory drive (standard state: decerebrate with intact descending neuromodulatory systems vs. minimal state: pentobarbital with said systems significantly inhibited). Fast amplification factors averaged 2.0 +/- 0.7 (mean +/- SD) in the standard neuromodulatory state. That is, the effective synaptic current was nearly twice as large at its peak as it was at hyperpolarized levels, ranging as high as 2.6. Although fast amplification was often smaller than the amplification of steady inputs, the difference was not statistically significant. However, the voltage at which fast amplification began was approximately 10 mV more depolarized (P < 0.01). It is concluded that both dynamic and steady inputs can be amplified, but there may be differences in mechanism.