It has been hypothesized that lax vowels may be relatively unaffected by dysarthria, owing to the reduced vocal tract shapes required for these phonetic events (G. S. Turner, K. Tjaden, & G. Weismer, 1995). It also has been suggested that lax vowels may be especially susceptible to speech mode effects (M. A. Picheny, N. I. Durlach, & L. D. Braida, 1986). Studies evaluating these suggestions are lacking, however. The present study was an initial investigation of the vowel space area formed by the lax vowels /[IOTA]/, /[epsilon]/, and /[upsilon]/, occurring in a passage read in habitual, fast, and slow conditions by speakers with amyotrophic lateral sclerosis (ALS), speakers with Parkinson's disease (PD), and healthy controls. Vowel space areas for speakers with ALS but not speakers with PD differed from those for the appropriate control group. Thus, only the results for the PD group support the hypothesis that lax vowel space areas for speakers with dysarthria should be similar to those for neurologically normal talkers. Compared with the habitual condition, rate reduction was associated with an expanded vowel space area for all of the healthy talkers but for only about half of the speakers with dysarthria. In addition, about half of the speakers in each group demonstrated a reduced vowel space area for the fast condition relative to the habitual condition, although the statistical analyses indicated no difference in vowel space areas for the fast and habitual conditions. The current study therefore provides only limited support for the idea that lax vowels are highly susceptible to speech mode effects, at least when the speech modes under consideration include faster-than-normal and slower-than normal rates. Clinical implications are discussed.