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Native speakers of a language can detect quite small departures from "correct" timing in the production of a sentence, which implies that they "understand" the rules used by the speaker to determine the timing of his articulations. In order to find out what the temporal constraints are on the speaker, an experiment was designed to measure the just noticeable difference (JND) for phoneme duration in natural speech. The stimuli were produced by an electronic equivalent of tape cutting and splicing. The incremental and decremental JND's were measured for the duration of the closure interval of a/p/, in initial position and in intervocalic position, in a stressed and an unstressed syllable, and for the duration of, if necessary, a stressed and an unstressed vowel. JND's were also measured for stimuli in which the duration of a stop and an adjacent vowel were altered simultaneously in opposite directions. 1) The results are not compatible with Weber's law. 2) Subjects were generally more sensitive to changes in vowel duration than to changes in stop-closure duration. 3) Subjects were no more sensitive to changes in the duration of a phoneme when the change was compensated in the following phoneme than when it was not compensated.