In female sheep fetuses, the mesonephros and genital ridge can be identified at days 20 and 23 of gestation (term = 145 days), respectively. Moreover oogonia can be observed at the genital ridge from as early as day 23. Around day 55 of gestation, some germ cells enter meiosis coincident with the arrival of mesonephric-derived somatic cells (i.e. the rete ovarii). From day 75, 100, 120 and 135 of gestation, primordial (one layer of flattened granulosa cells), primary (one complete layer of cuboidal granulosa cells; early preantral), secondary (preantral) and tertiary (antral) follicles, respectively, develop within the innermost regions of the ovarian cortex. During the early neonatal period highly variable numbers of antral follicles may be present. After examination of Booroola fetuses from day 28 of gestation, it seems that the FecBB gene is associated with retarded development of the heart (day 28) mesonephros (days 30-40) and from day 30 to early neonatal life, the ovary. With respect to the ovary, fewer oogonia (days 30-40), primordial follicles (day 75-90) and growing follicles (day 120 to 6 weeks after birth) have been observed in females carrying the FecBB gene. By contrast, the FecBB gene is not associated with differences in plasma gonadotrophin or immunoreactive inhibin until early neonatal life. In Inverdale (I) fetuses heterozygous for the FecXI gene (I+), retarded germ cell development was observed at days 40 and 90 of gestation. In putative homozygous carriers (II) of the Inverdale gene, germ cell development appeared normal until day 100, but thereafter from day 120 normal secondary follicles were not observed, although many abnormal follicular-like structures were present. In both I+ and II fetuses no obvious differences in gonadotrophin concentrations have been noted. Collectively, the evidence suggests that the fecundity genes FecBB and FecXI, which affect ovulation rate in sexually mature females, are regulating organ differentiation or germ cell maturation or both processes during fetal life.