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1. Neuronal activity bearing a temporal relationship with spontaneous reticulo-ruminal movements was recorded with micro-electrodes from the medulla oblongata in halothane-anaesthetized sheep. Recording sites were located histologically after causing electro-coagulation at the micro-electrode tip.

2. One hundred and forty-four gastric units were recorded from the dorsal vagal nucleus and up to 1 mm dorsal and lateral to the nucleus between transverse planes 1 mm caudal, and 4 mm rostral, to the obex. It is considered that records were obtained from the regions of cell bodies.

3. The discharges of thirty-two vagal preganglionic motoneurones were identified by an antidromic collision technique. Conduction velocities ranged from 10-26 m/sec. They were located in the dorsal vagal nucleus and up to 0·5 mm dorsal and lateral to the nucleus. The majority of motoneurones innervated either the reticulum or the rumen. One ruminal unit discharged during both primary and secondary cycle movements.

4. One hundred and twelve units which were not orthodromically or antidromically activated by stimulating the vagus nerves were considered to be interneurones. Four types were distinguishable on the basis of their patterns of discharge during primary cycle movements.

5. The discharges of Type A interneurones resembled those of gastric motoneurones, having no resting discharge between contraction cycles. Their discharges were temporally related to either reticular contractions or rumen contractions during primary and secondary cycle movements.

6. Types B and C interneurones have resting discharges which, respectively, increased and either decreased or stopped during each primary cycle movement.

7. Discharges of only three units identified as interneurones resembled the discharges of gastric vagal afferent units.