The purpose of the current study was to determine the extent to which a clear speech instruction would modulate acoustic measures of intonation and speech timing in speakers with and without PD in reading and extemporaneous speech tasks. Speakers both read simple reading passages and responded to self-selected extemporaneous speech prompts. Participants first performed the reading and extemporaneous speech tasks in a habitual manner, and then a second time in a clearer than usual style following an instruction to "over-enunciate each word." The speech samples were parsed using a process specified by the Systemic Theory of Functional Linguistics and further analyzed acoustically for measures of intonation and speech timing. Results revealed that speakers with PD presented with overall less variation in fundamental frequency and slower falling and rising fundamental frequency contours than controls regardless of speaking style. Speakers with PD exhibited significantly less clarity-related reduction in articulation rate than controls for both speaking tasks. For the reading task, all speakers exhibited clarity-related reduction in articulation rate. For the extemporaneous task, only control speakers exhibited clarity-related changes in articulation rate and pause durations, whereas speakers with PD exhibited no change in articulation rate between the habitual and clear speech styles. These results may indicate that speakers with PD exhibited a less robust clear speech response than controls. Additionally, it is possible that the higher cognitive-linguistic load of the extemporaneous task interfered with the magnitude of the clear speech response. Together, the demands of generative language and clear speech production associated with the extemporaneous task may have reduced some of the clear speech benefits for speakers in the PD group.