Summary -The aim of this study was to establish the importance of genetic, biological and environmental factors which may affect sex ratio at birth in dairy cattle. Four main kinds of variations may be considered: heterogeneity between the mothers with respect to the probability of a male birth, variation in this probability according to parity within the progenies; correlation between the sexes of adjacent births, and breeders' decisions. We developed models in which all or some of these factors were simultaneously included, and tested them on a sample of about 266 000 dams. The male birth proportion was found to be significantly heterogeneous between dams, increasing over time within progenies, and influenced by the breeder's choice of stopping or continuing the dams' reproductive career. The