I have previously suggested that parental levels of several hormones (gonadotrophin, oestrogen, testosterone) at the time of conception affect the sex of mammalian offspring (James, 1986, J. theor. Biol. 118, 427). In this note it is suggested that progesterone also has such an effect. Clutton-Brock & Iason (1986, Q. Rev. Biol. 61, 339) concluded that variation of sex ratio had been reasonably well established with a number of variables in a number of non-human mammalian species. In this note, discussion centres on the adequacy of my hypothesis to accommodate these and other data. The evidence now is strong that hormonal variation is associated with variation of sex ratio at birth in some mammalian species: in a few species (e.g. man and the vole) there is a strong presumption that the relationship is causal. However the same sort of cause apparently affects the sex ratio in opposite directions in different species: so it may be speculated that the same hormone has opposite effects on sex ratio in different species.