Human and nonhuman primates commonly alter the configuration of their lips during vocal production and thereby modify vocal tract length and shape. In nonhuman primates, however, the effects of lip configuration on call structure are unknown. This study was designed to investigate the importance of lip configuration in rhesus monkey (Macaca mulatta) vocal production by temporarily blocking lip movement with injections of xylocaine. For "coo" vocalizations, an affiliative contact call that is normally produced with protruded lips, the xylocaine treatment had no statistically significant effect on call duration or characteristics of the fundamental frequency (i.e., features associated with respiration and laryngeal function). However, the two formant frequencies (i.e., features resulting from the filtering properties of the supralaryngeal cavity) of the call were significantly effected. Specifically, formant frequencies increased, as would be expected from an individual incapable of compensating for a shortened vocal tract. In contrast to coos, xylocaine had no statistically significant effect on the acoustic structure of "noisy screams," a call given in response to being attacked by a dominant and produced with retracted lips (i.e., a shortened vocal tract). Results suggest that for some vocalizations, lip configuration may be essential for achieving the intended acoustic target, whereas for other vocalizations, it is less important.