This study used glossometry to examine the position of the tongue and the velocity of its movements in vowels spoken normally and at a self-selected fast rate. The subject in experiment 1 showed lingual undershoot for stressed vowels in “a bib again” and “a bob again.” The tongue was lower for /i/ and higher for /a/ at the fast rate than at the normal rate. The stressed vowels exerted an effect on unstressed vowels: The tongue was lower in the schwas that preceded and followed /a/ than /i/. Only one of the three subjects in experiment 2 showed no lingual undershoot for fast-rate /i/. The tongue was higher at the fast rate than at the normal rate in the schwas flanking /i/ so that the displacement was less at the fast rate than at the normal rate. Another talker increased the peak velocity of tongue movements at the fast rate and showed no undershoot for /a/. Multiple regression analyses showed that the timing of movements for successive phonetic segments accounted well for undershoot in only one of the three subjects. The results suggest that in order to model the effects of speaking rate on the tongue movements used in forming stressed vowels, it will be necessary to take into account: (1) how much vowels are shortened at a fast rate; (2) how much the peak velocity of tongue movements is increased, if at all; and (3) the position of the tongue before and after the stressed vowels. All three factors are likely to be influenced by how clearly the talker wishes to speak. © 1988, Acoustical Society of America. All rights reserved.