The relation between impedance change and the location and magnitude of a tonic synaptic conductance (Fox 1985) was examined in compartmental motoneuron models based on previously published data (Cullheim et al. 1987; Fleshman et al. 1988). The dependence of motoneuron impedance on system time constant, , electrotonic length, L, and dendritic-to-somatic conductance ratio, , was examined, showing that the relation between impedance phase and differed markedly between models with uniform and nonuniform membrane resistivity. Dendritic synaptic conductances decreased impedance magnitude at low frequencies; at higher frequencies, impedance magnitude increased. The frequency at which the change in impedance magnitude reversed from a decrease to an increase - the reversal frequency, F_r - was a good estimator of electrotonic synaptic location. A measure of the average normalized impedance change at frequencies less than F_r, cuZ, estimated relative synaptic conductance. F_r and cuZ provided useful estimates of synaptic location and conductance in models with nonuniform (step, sigmoidal) and uniform membrane resistivity. F_r also provided good estimates of spatial synaptic location on the equivalent cable in both step and sigmoidal models. Variability in relations between F_r, cuZ and conductance location and magnitude between neurons was reduced by normalization with and . The effects on F_r and cuZ of noise in experimental recordings, different synaptic distributions and voltage-dependent conductances were also assessed. This study indicates that location and conductance of tonic dendritic conductances can be estimated from F_r, cuZ, and basic electrotonic motoneuron parameters with the exercise of suitable precautions.