Five dogs were hypophysectomized on Day 4 and 9 on Day 18. Prolactin and LH stimulation tests showed that hypophysectomy was complete in 6 dogs only. In these dogs, the progesterone concentration was measured in the peripheral blood; it decreased sharply immediately after surgery. It regained normal values in 3 of the 4 dogs hypophysectomized on Day 4, and remained low in the 2 dogs hypophysectomized on Day 18. This indicates that, in the dog, luteal function is autonomous during a certain period. The luteal period of the 3 dogs hypophysectomized on Day 4 was shorter than that of control animals, although the time of onset of luteal regression appeared to be similar. This indicates that pituitary luteotrophic support is required during the second part of the oestrous cycle of the dog.