1. Sacral parasympathetic preganglionic neurones were identified by intracellular and extracellular micro-electrode recording of antidromic potentials in response to stimulation of the pelvic nerve or the second or third sacral ventral roots.2. The segmental distribution of autonomic neurones varied in different cats. In some cats they were mainly in S2 segment, in others in S3 and in the remainder, in both S2 and S3.3. The antidromic potentials showed initial segment-somadendritic (IS-SD) inflexions and delayed depolarizations and were slightly less prolonged than those of sympathetic neurones but more prolonged than those of spinal motoneurones. After-hyperpolarization was observed after the antidromic spike potential.4. The conduction velocities for sacral parasympathetic preganglionic fibres were less than 12.5 m/sec and thus were similar to those of sympathetic preganglionic fibres.5. Parasympathetic neurones were not excited by micro-electro-phoretically applied 5-hydroxytryptamine, noradrenaline or acetylcholine.