To study the coordination between the masticatory and extrinsic tongue muscles during natural chewing, electromyographic activities in the digastric (Dig) as a jaw opener, the masseter (Mas) as a jaw closer, the genioglossus (Gg) as a tongue protruder, and the styloglossus (Sg) as a tongue retractor as well as jaw movement trajectories were recorded while rabbits chewed soft. hard, and very hard foods. The Dig and Gg were active in the jaw-opening phase (OP active group), and the Mas and Sg were active in the jaw-closing phase (CL active group). Food consistency affected differently on the duration of burst activities between the muscle groups, i.e. in the CL active group, the duration was longer for the harder food, while there was no difference in the duration of the OP active group among the foods. During hard food chewing in particular, we confirmed our recent findings that reflexly-induced short but large bursts of activity could be documented in the Dig during the jaw-closing phase. Similar short bursts were also documented in the Gg as with the Dig in this study. Inhibitory periods were often observed in the Mas with the Dig short burst and were also observed in the Sg along with the Gg short burst: however the inhibitory effect in the Sg was less pronounced. These findings suggest that: (1) both masticatory and extrinsic tongue muscles are active in a well-coordinated manner during stable chewing, but that (2) reflex effects on antagonistic muscles (i.e. Dig vs. Mas in the masticatory muscles, Gg vs. Sg in the tongue muscles) evoked by tooth contact during chewing may not be analogous between the two muscle groups. (C) 2002 Elsevier Science B.V. All rights reserved.