This study examined the relationship between hypothalamically elicited emotional behaviors and electromyographic activity in the masseter muscle of the cat. Electromyographic amplitudes resulting from stimulation at hypothalamic sites at which affective defense and quiet biting attack behaviors were elicited were compared with those recorded during stick biting that simulated mastication. The electromyographic activity elicited by hypothalamic stimulation was greater than that found from stick biting for all behavioral sites. At control sites, from which no behavior could be elicited, the electromyographic activity associated with stick biting exceeded that observed from the hypothalamic sites. These findings suggest a relationship between hypothalamically elicited behaviors and increased levels of jaw muscle activity.