Voice onset time (VOT) is a salient acoustic parameter of speech which signals the 'voiced' and 'voiceless' status of plosives in English (e.g. the initial sound in 'bat' versus the initial sound in 'pat'). As a micro-temporal acoustic parameter, VOT may be sensitive to changes in hormones which may affect the neuromuscular systems involved in speech production. This study adopted a novel approach by investigating the effects of menstrual cycle phase and sex on VOT. VOT data representing the six plosives of English (/p b t d k g/) were examined for seven women (age 20-23 years) at two phases of the menstrual cycle (day 18-25: high estrogen and progesterone; day 2-5: low estrogen and progesterone). Results indicated that menstrual cycle phase had a significant interaction with the identity of the plosive (F (5,30) = 5.869, P < 0.002). Menstrual cycle phase also had significant effects on the contrasts between cognate voiced and voiceless plosives (F (1,6) = 11.444, P < 0.02); samples from the high hormone phase displayed an enhanced voiced/voiceless contrast. Subsequently, VOT data samples from the two phases of the menstrual cycle were compared with those from five men in order to explore sex differences at different phases of the menstrual cycle. Low hormone phase samples displayed no significant sex differences for either VOT values (F (1,10) = 2.085, P > 0.05), or the contrast between voiced and voiceless cognates (F (1,10) = 0.407, P > 0.05). In contrast, the high hormone phase VOT samples displayed significant plosive by sex interactions (F (5,50) = 4.442, P < 0.005). In addition, significant sex differences were found for the contrasts between cognate voiced and voiceless plosives (F (1,10) = 5.019, P < 0.05); the women displayed a more marked voiced/voiceless contrast. The findings suggest that ovarian hormones play some role in shaping some temporal components of speech.