The speech rate is one of the important prosodic parameters essential for the naturalness of an utterance, yet comparatively little is known on the fine structures of speech rate variation in natural utterances. On the basis of the authors' definition of the relative local speech rate, the present paper describes an analysis of the changes in the local rate of speech units, each produced in isolation, when they are embedded in connected speech. The results, together with those already obtained by the authors, will lead to a complete scheme for speech rate control in speech synthesis by concatenation.