This study was designed to investigate whether stimulation of the mesencephalic reticular formation (MRF) induces rhythmic jaw movements (RJMs) and, if it does so, to determine the RJM-inducing region in the MRF in ketamine-anesthetized guinea pigs. The results were as follows: (1) Repetitive electrical stimulation of the MRF at the level of the red nucleus induced rhythmic EMG bursts in the anterior digastric muscle (DIG). (2) The duration and cycle time of the rhythmic DIG EMG burst induced from the medial MRF were longer than those induced from the lateral MRF. (3) Repetitive MRF stimulation after paralysis still induced rhythmic multiple-unit activities in the anterior digastric motoneuron pool. (4) Neither precollicular decerebration nor cerebellectomy affected the MRF induction of RJMs. (5) Transverse hemisection at the rostral border of the pons abolished the RJMs induced from the contralateral, but not ipsilateral, MRF. Midline section of the midbrain abolished RJMs induced from the MRF on either side. (6) A lesion in the pontine pyramidal tract abolished the RJMs induced by stimulation of the ipsilateral cortical masticatory area (CMA), but not those induced from the contralateral MRF. (7) A unilateral lesion of the oral portion of the gigantocellular reticular nucleus, where the rhythm generator for the CMA-induced RJMs is located, abolished RJMs induced from not only the CMA, but also MRF on the contralateral side. (8) Microinjection of L-glutamate into the lateral, but not medial, MRF induced RJMs similar to those elicited by repetitive electrical stimulation of the same site.(ABSTRACT TRUNCATED AT 250 WORDS)