Pregnant mares' serum gonadotrophin (PMS) when added to the incubation medium of slices of ewes' corpus luteum increases the amount of progesterone liberated. The activity of the luteal tissue under the described in vitro conditions varies with the age of the corpus luteum, in a similar manner as under normal in vivo conditions. On the contrary, the sensitivity of the tissue to PMS stimulation in vitro increases during the degenerative phase of the corpus luteum.