Tonic activity of the external urethral and anal sphincter muscles is necessary to maintain urinary and fecal continence. Excitatory descending and segmental synaptic inputs facilitate the activity of the sphincters during continence. However, intrinsic properties of the sphincter motoneurons may also contribute to the tonic activity of the sphincters. It is hypothesized that sphincter motoneurons will have the ability to amplify synaptic input and respond with prolonged firing or maintained depolarizations (non-linear responses) to excitatory afferent stimulation and depolarizing current injection Intracellular recordings with QX-314 showed that half of the sphincter and hindlimb motoneurons in the first sacral segment of the decerebrate cat displayed non-linear responses to current ramp injection and/or sustained depolarization following depolarizing current pulses. During the non-linear response, an increase in membrane conductance, along with an increase in synaptic noise, was observed. Prolonged firingof the sphincter efferents could be elicited by excitatory afferent stimulation during bladder filling. Phasic increases in activity were observed in the sphincter electroneurograms along with an increase in the tonic activity and prolonged firing of individual units. It is proposed that the non-linear membrane response observed in sphincter motoneurons may be a mechanism to promote tonic activity of the sphincter efferents and help to maintain continence. (Abstract shortened by UMI.)