The plateau potential, responsible for the bistable state of spinal motoneurons, recently described in the decerebrate cat, was suggested to depend on serotonin (Hounsgaard et al. 1984). In an in vitro preparation of the spinal cord of the turtle we now show that serotonin, applied directly to the bath, transforms the intrinsic response properties of motoneurons, uncovering a plateau potential and voltage sensitive bistability. The changes induced by serotonin were blocked by Mn⁺⁺, while the plateau potential and the bistability remained after application of tetrodotoxin. We conclude that serotonin controls the expression of a Ca⁺⁺ dependent plateau potential in motoneurons.