The current-chop technique has shown that I-V characteristic of the membrane are nonlinear in lumbar motoneurons of isolated perfused frog spinal cord. Input resistance of the membrane decreased with depolarization when constant current was applied during 0.1-1.0 s. However, injection of current 40-60 nA during 1-2 min led to an increase of the membrane resistance to the initial value. As a result the membrane potential could be shifted to the positive level up to +50 mV and more. Monosynaptic excitatory postsynaptic potentials evoked by stimulation of the brainstem or by microstimulation of ventrolateral tract fibres were found to reverse completely at a positive level of the membrane potential. In most cases the reversal potentials ranged between 0 mV and -10 mV.