A method of adjustment to internal criterion was used to investigate the perception of consonant duration. Three adults and two children, 5 and 7 years old, adjusted the durations of /s/ and /t/ in the synthesised wordssay, tay, stay, andstray. The words were presented in two synthesised sentences, in both an adult male and a child's voice. The perceptual data were compared with speech produced by the same and different subjects. The just noticeable difference (jnd) for consonant segment duration was of the order of 25 msec; but the internal criterion for “acceptable” durations can shift over time by more than the jnd at any one time. Durations of /s,t/ in perception and production were strongly associated within and between subjects, but there were some interesting adult—child differences. Compared with adults, children appeared to lack sensitivity to differences between sentence contexts in the same speaker voice, but to differentiate more between /s,t/ durations they required for the child versus the adult speaker.The differences required in perception for the child speaker did not always mirror the child listener's own production durations. These data are interpreted with respect to possible processes governing the regulation of segmental timing and rhythm in perception and production. [Supported by SRC, Great Britian.]