A threshold‐tracking method was used to measure both the incremental and the decremental just noticeable differences for segment duration in naturally spoken sentences. The measurements were made for a /p/ in five different contexts, including two that were word initial, two that were word medial, and one that was word final; for /∫,m/, and /l/ all in initial prestress position; and for a stressed vowel /ɔ/. The results show that subjects are much more sensitive to changes in vowel duration than to changes in consonant duration, and that changes in segment duration may have several different perceptual effects, including changes in perceived stress and perceived rhythm. When subjects based their judgments on changes in perceived stress or rhythm, they were usually able to detect smaller changes in duration than when they attended to other aspects of the stimuli.