There is an active interest in torpor-like states for translational applications and the neural basis of torpor is presently a hot topic of research. Activation of Esr1-expressing preoptic area neurons generates torpor-like states, suggesting a link between hormonal status and torpor. This is an important consideration for the fundamental biology as well as translational applications, as gonadal hormones fluctuate greatly between, and within, individuals. The aim of the present study was to investigate the roles of sex and oestrus cycle / oestradiol (E2) in modulating mouse torpor tested across three studies: 1) comparing torpor between sexes (n=16), 2) comparing torpor in females across the oestrous cycle (n=8) and, 3) examining the effect of exogenous E2 on torpor (n=16). To induce torpor, adult C57BL6j mice were housed at 21°C and fasted for 24 h, during which their surface temperature was imaged. Consistent with findings in other species, female mice displayed cooler and longer torpor bouts (p<0.01). Over the oestrus cycle, within-animal comparisons revealed that females in dioestrus had longer, cooler bouts of torpor compared to oestrus (p<0.01) and proestrus (p<0.05). E2 administration led to longer bouts in females (p<0.05) which was not observed in males. These data suggest that hormonal status strongly modulates torpor bouts in mice. It remains to be determined whether this effect is mediated at the level of the hypothalamus, and the extent to which oestradiol signalling is necessary for torpor.