When listening to someone's voice: what stimulus duration is required to tell whether the person speaking is a man or a woman; what are the acoustic cues in speech that influence such judgements; and how does manipulations in these acoustic cues influence such judgements? The vowels of five men and five women were recorded and played at a number of brief durations. The vowels were either unmodified and thus differed in both glottal-pulse rate and vocal-tract length (Expt 1), or had their glottal-pulse rate modified to be the same (Expt 2), or had their simulated vocal-tract length modified to be the same (Expt 3). Listeners were required to indicate whether the vowels were spoken by a man or woman. Results show that correct speaker-sex judgement requires only brief duration stimuli (about 10 ms), and that the removal of either the glottal-pulse rate or the vocal-tract length cue leads to reduced performance in judging the sex of the original speaker.