1. Passive electrical properties, afterpotential properties and the pattern of repetitive discharge induced by constant current injection were studied in axotomized lumbar motoneurones. 2. Following axotomy, the motoneurones showed a larger input resistance and membrane time constant, but had a normal electrotonic length. 3. Duration and peak amplitude of the afterhyperpolarization (ahp) were on average unchanged following axotomy. There was, however, a significant reduction in the conductance underlying the ahp. The distribution of values for ahp duration was also narrower following axotomy, with an absence of long and short values. 4. As in normal motoneurones, the ahp conductance, calculated from the voltage, decayed in an approximately exponential manner with a phase of slower decay corresponding to the hyperpolarizing phase of the ahp. The phase of slower decay was, however, less accentuated and several axotomized motoneurones showed an exponential decay of the ahp conductance. 5. The frequency--current (f--I) curves for the first interspike intervals were, as in normal motoneurones, non-linear, deviating upwards at higher frequencies. The steady-state f--I relations were, however, linear in most of the axotomized neurones. The slopes of the f--I curves were steeper following axotomy. These steeper slopes were well correlated with the decreased ahp conductance. 6. The interspike voltage trajectories were similar to those in normal motoneurones, i.e. concave at low current strength and changing to a convex shape with increasing current injection. The changes in the trajectory shape were not correlated with the changes in the slope of the f--I curves. 7. It is concluded that the afterhyperpolarization conductance is the major factor in the regulation of repetitive firing in axotomized motoneurones.