sex differences in critical flicker frequency (cff) have been found by hartmann (1934) and ginsburg et al. (1982), with men appearing to be more sensitive than women* the nature of these sex differences has been questioned. ginsburg et al. (1982) have suggested that these differences may be due to hormonal influences that could be further explored by studying cff across the menstrual cycle. it has been proposed by bern (1974) that sex role perception might be a variable involved in sex differences* the present study was designed to explore the sex difference in cff and two interpretations of it;.: the influence of hormones and the influence of sex role perception. the former variable was investigated by looking for changes in sensitivity during the menstrual cycle, since any such found would imply a hormonal basis. the latter variable was studied thru the use of the bern sex role inventory, which includes both femininity and masculinity.scales. two experiments were conducted using a cff task based on the method of limits. in experiment i, three female volunteers with menstrual regularity were tested three times a week for two complete menstrual cycles. in experiment ii, 30 male and 33 female subjects were tested once on the cff task and the bern inventory. in addition. all females answered a questionnaire concerning their menstrual cycle* a significant sex difference in cff was found, with men more sensitive* ho clear relationship was found between cff and the menstrual cycle, either in subjects tested repeatedly over the cycle or in the larger group tested once# sex role perception showed no relation to cff among females. however, among males some relationship was observed which suggests further study.