This study investigates how the five lexical tones in Thai are realized on primary-, secondary-, and unstressed syllables produced at fast, normal and slow rate. The results revealed that 1) speech rate does not have any significant effect on F0 height, excursion size and F0 peak and valley locations of Thai tones, 2) tones on primary-stressed syllables have a larger excursion size than those on secondary- and unstressed syllables, and 3) the five-way tonal contrast in the language is maintained regardless of changes in speech rate and stress.