The recent studies of the anatomy of the eustachian tube and related structures in the Rhesus monkey (Macaca mulatto) have shown that the monkey tubal system is similar to the human. This investigation in Rhesus monkeys was an attempt to verify previous studies in other animals that the tensor veli palatini muscle was the only dilator of the eustachian tube. Two unipolar stimulating electrodes were introduced into the foramen ovale, and the mandibular branch of the trigeminal nerve was electrically stimulated. Simultaneously, pressure-flow recordings through the eustachian tube were monitored. Stimulus-response relationships were obtained for five Rhesus monkeys. The degree of tubal dilation by the tensor veli palatini muscle contraction was shown to be a function of stimulating current levels. Artificially induced dilations were quite similar to the physiological dilations during swallowing when these animals were tested alert. Following complete transection of the tensor muscle, regardless of the stimulus level, no tubal dilations were observed. Stimulation of the nerve to the internal pterygoid and stimulation of the levator veli palatini muscle induced only constrictions of the tube. The tensor veli palatini muscle is the only paratubal muscle responsible for active dilation of the eustachian tube in the Rhesus monkey, and its motor innervation is the mandibular division of the trigeminal nerve.