The problem of postpartum anoestrus is a real one because it results in prolongation of the time between calvings. The interval between the calving and resumption of cyclic ovarian activity depends on several factors, i.e., amount of feeding before and after parturition, level of milk yield, age of the animal, calving difficulty, presence of a bull in the herd, season and its photoperiodism and particularly the suckling or lactating status of the cow. The anoestrus period is longer in suckled cows (30 to 110 days) than in milked cattle (20 to 70 days). The physiology of the hypothalamic-pituitary-ovarian axis is still far from clear. Nevertheless, some events are very well demonstrated. The pulsatile release of LH and GnRH and the pituitary sensitivity to GnRH increase gradually after calving. They are inhibited by suckling, which acts more on LH and GnRH release than on their synthesis. Suckling or the presence of a calf can exercise its action via oestrogens. Suckling inhibits oestrogen synthesis by follicular cells and diminishes their feed-back positive effect on the hypothalamic-pituitary axis. The suckling effect depends on oestrogen concentrations and on time after calving. The progressive LH release induces the synthesis of progesterone. After calving, the first luteal phase is shorter and the progesterone plasma concentrations are lower than what is observed during a normal cycle. Amongst some hypotheses proposed, premature luteolysis induced by uterine prostaglandins offers a new and very interesting field of research related to the utero-ovarian relationship after calving. The effects of FSH, prolactin and glucocorticoids hormones are much less understood.