some propositions on male and female sexual orientation will be considered. some of these are established; others are more speculative. the aim is to offer some notes towards a coherent, comprehensive theory of sexual orientation. 1. the distinction between butch and femme lesbians seems real rather than a social construct. 2. high levels of prenatal steroid hormones seem to be causally associated with the sexual orientation of butch lesbians. however it is not established whether the causal process operates prenatally or postnatally (or both). this is so because prenatal hormone levels are thought to correlate positively with postnatal hormone levels. and high postnatal hormone levels may facilitate homosexual behaviour as a consequence of sensation-seeking. 3. male bisexuals also are interpreted to have been exposed to high prenatal testosterone levels. but (for reasons similar to those outlined above in regard to butch lesbians) it is unclear whether these have a direct prenatal effect on the brain or whether they are precursors of high postnatal testosterone levels, which are associated with male bisexual orientation by promoting sensation-seeking behaviour. 4. postnatal learning processes seem to be causally involved in the sexual orientation of some femme lesbians and some exclusive male homosexuals. 5. some homosexual men have genes that predispose to their sexual orientation. 6. the same may apply to some lesbians, but such genes have not, as far as i know, been identified. 7. people (of both sexes) who engage in same-sex sexual behaviour may be classified simultaneously in two ways, viz (1) ‘active’ vs ‘passive’ and (2) those who do and those who do not engage (or consider engaging) in sex with members of the opposite sex. ex hypothesi, some of the ‘active’ ones initiate some of the ‘passive’ ones. the active ones are driven more by hormones and the passive ones by psychosocial factors. the active males contain a substantial proportion of self-identified bisexuals; and the active females a substantial proportion of self-identified butches. 8. these two active categories (butch lesbians and male bisexuals) share a number of endocrinological, psychological, morphological and behavioural features vis-à-vis their exclusively homosexual and heterosexual peers. methods of testing some of these ideas are presented.