Effects of speaking rate on the production of lexical tones (mid, low, falling, high, rising) were investigated in Thai. Stimuli consisted of bisyllabic adverbials (first syllable unstressed, last syllable stressed) elicited in a fixed syntactic and prosodic environment. For unstressed and stressed syllables separately, F₀- and time-normalized, fitted, third-order polynomial curves were used to compare height and slope characteristics of the tonal contours at each of 11 measurement locations between fast and slow speaking rates. Results indicated that speaking rate effects on F₀ contours of unstressed syllables are more extensive, both in terms of height and slope, than those of stressed syllables. In particular, the height of F₀ contours in unstressed syllables was generally higher in the fast speaking rate when compared to the slow. Analysis of the preceding carrier syllable revealed that changes in height of F₀ contours of unstressed syllables may be due primarily to perseverative effects of tonal coarticulation rather than to stress itself. The slope of F₀ contours in unstressed syllables varied depending on range of F₀ movement. Thai tones with substantial F₀ movement (falling, high, rising) exhibited overall flatter slopes at the fast speaking rate; those tones with lesser F₀ movement (mild, low) displayed steeper slopes. Despite extensive changes in height and shape, the five-way tonal contrast appears to be maintained in unstressed syllables at a fast speaking rate albeit in a different tonal space.