X-chromosome inactivation ensures dosage compensation for X-linked genes products in mammals. In eutherian mammals, X inactivation is controlled by the non-coding Xist transcript and is believed to be random, with either the paternal or maternal X chromosome being chosen for silencing. In mice, where most studies on the developmental timing of X inactivation have been conducted, Xist expression is initially imprinted and only the paternal X chromosome is inactivated during early mouse development and in extraembryonic tissues. This imprinted form of X inactivation is reversed in the inner cell mass, after which random X inactivation takes place in embryonic lineages. Whether this complex pattern of Xist expression and X-inactivation dynamics exists in other mammals remains unclear. Indeed, the degree to which the X-inactivation process is conserved between different mammals remains an open question. For example, marsupials have no Xist gene and show exclusive paternal X inactivation in all lineages.We have investigated X inactivation during early embryogenesis in other mammals such as rabbits, and find that the kinetics and monoallelic regulation of X inactivation are very different to the situation in mice. Our results suggest that although some molecular mechanisms are shared, there is remarkable evolutionary diversity in the developmental events underlying dosage compensation in different mammals.