Both rate reduction and increased loudness reportedly are associated with an increase in the size of the articulatory-acoustic working space and improved acoustic distinctiveness for speakers with dysarthria. Improved intelligibility also has been reported. Few studies have directly compared rate and loudness effects for speakers with dysarthria, however, although rate reduction and increasing vocal loudness are common treatment techniques. In the current study, 15 speakers with dysarthria secondary to multiple sclerosis, 12 speakers with dysarthria secondary to Parkinson's disease (PD), and 15 healthy controls read a passage in habitual, loud, and slow conditions. Rate and loudness variations were elicited using magnitude production. Acoustic measures included articulatory rate, sound pressure level, vowel space area, first moment difference measures, and F2 trajectory characteristics for diphthongs. Ten listeners scaled intelligibility for reading passages produced by the speakers with dysarthria. Relationships between intelligibility estimates and acoustic measures were determined by regression analysis. All speaker groups reduced articulatory rate for the slow condition and increased vocal intensity for the loud condition, relative to the habitual condition. Vowel acoustic distinctiveness, as indexed by vowel space area, was maximized in the slow condition, but stop consonant acoustic distinctiveness, as indexed by first moment difference measures, was maximized in the loud condition. F2 slope measures for diphthongs were not consistently affected by rate or loudness. Scaled intelligibility for speakers with PD also improved in the loud condition relative to both the habitual and slow conditions. Intelligibility estimates for speakers with dysarthria, however, were not strongly related to acoustic measures of supraglottal behavior. Findings are compared with previous studies, and hypotheses for future treatment studies are discussed.KEY WORDS: dysarthria, rate, loudness, acoustics, intelligibility