To determine how trigeminal brainstem interneurons pattern different forms of rhythmical jaw movements, four types of motor patterns were induced by electrical stimulation within the cortical masticatory areas of rabbits. After these were recorded, animals were paralyzed and fictive motor output was recorded with an extracellular microelectrode in the trigeminal motor nucleus. A second electrode was used to record from interneurons within the lateral part of the parvocellular reticular formation (Rpc-␣, n ϭ 28) and ␥-subnucleus of the oral nucleus of the spinal trigeminal tract (NVspo-␥, n ϭ 68). Both of these areas contain many interneurons projecting to the trigeminal motor nucleus.The basic characteristics of the four movement types evoked before paralysis were similar to those seen after the neuromuscular blockade, although cycle duration was significantly decreased for all patterns.Interneurons showed three types of firing pattern: 54% were inactive, 42% were rhythmically active, and 4% had a tonic firing pattern. Neurons within the first two categories were intermingled in Rpc-␣ and NVspo-␥: 48% of rhythmic neurons were active during one movement type, 35% were active during two, and 13% were active during three or four patterns.Most units fired during either the middle of the masseter burst or interburst phases during fictive movements evoked from the left caudal cortex. In contrast, there were no tendencies toward a preferred coupling of interneuron activity to any particular phase of the cycle during stimulation of other cortical sites. It was concluded that the premotoneurons that form the final commands to trigeminal motoneurons are organized into subpopulations according to movement pattern.Key words: rhythmical movements; pattern generation; mastication; trigeminal system; brainstem; rabbit To be effective, motor patterns need to be adapted to the needs of the organism. As a result, the same body parts often participate in several stereotyped behaviors that differ only in the ordering and scaling of commands to the same pools of motoneurons (e.g., walking vs running and chewing on the right vs chewing on the left). The way in which neural circuits generate these different patterns has been investigated in many species, particularly in invertebrates, and evidence has been found that three basic forms of architecture exist: dedicated circuitry, distributed circuitry, and reorganizing circuitry (for review, see Morton and Chiel, 1994). It is probable, however, that most of the circuits controlling complex movements have features of more than one of the basic models.Dedicated circuits can generate only one pattern, and when they are triggered by a sensory input, they suppress other ongoing behaviors. The circuit that generates wing retraction in the mollusk Clione limacina seems to be an example of this type (Morton and Chiel, 1994). Reorganizing circuits generate different patterns when the effectiveness of synaptic connections between members of the total population of neurons changes. The key feature...