We investigated summation of steady excitatory and inhibitory inputs in spinal motoneurons using an in vivo preparation, the decerebrate cat, in which neuromodulatory input from the brainstem facilitated a strong persistent inward current (PIC) in dendritic regions. This dendritic PIC amplified both excitatory and inhibitory synaptic currents 2- to 3-fold, but within different voltage ranges. Amplification of excitatory synaptic current peaked at voltage clamp holding potentials near spike threshold (about -55 to -50 mV), whereas amplification of inhibitory current peaked at significantly more depolarized levels (about -45 to -40 mV). Thus, the linear sum of excitatory and inhibitory currents tended to vary from net excitatory to net inhibitory as holding potential was depolarized. The actual summed currents, however, diverged from the predicted linear currents. At the peak of excitation, summation averaged ~15% sub-linear (actual sum was less positive than the linear sum). In contrast, at the peak of inhibition, summation averaged ~18% supra-linear (actual more positive than linear). Moreover, these nonlinear effects were substantially larger in cells where the variation from peak excitation to peak inhibition for linear summation was larger. When descending neuromodulatory input was eliminated by acute spinalization, PIC amplification was not observed and summation tended to be either sub-linear or approximately linear, depending on input source. Overall, in cells with strong PICs, nonlinear summation of excitation and inhibition does occur, but this nonlinearity results in a more consistent relationship between membrane potential and the summed excitatory and inhibitory current.