Oral airflow and fundamental frequency (F0) data from Thai words beginning with a stop (T) or a stop + apical trill (Tr) are reported. A relatively higher F0 during voicing of the trill for Tr words than at vowel onset for T words was found. In addition, a greater rate of airflow was found for the trill in Tr words than for the vowel onset in T words. These results are interpreted to mean that the relatively higher F0 for trills is an automatic consequence of the high rate of airflow needed for apical trill production. It is proposed that this automatic F0 increase is the origin of a sound change found in dialects of Khmer in which an onset apical trill is lost, conditioning a falling-rising F0 contour as well as aspiration of the preceding stop.