The pattern of luteinizing hormone (LH) and prolactin release following the synchronisation of oestrus with progesterone or SC-9880 was studied. Of the 10 ewes treated with progesterone three did not exhibit oestrus. Where oestrus was synchronised with SC-9880 the preovulatory LH surge occurred at, or prior to, the onset of oestrus in two of five ewes. With progesterone the LH release occurred only after mating commenced. The average peak LH level in the plasma was 223,2, 147,6 and 93,7 ng/cm3 for the control, SC-9880 and progesterone treated ewes respectively. The differences were significant. A clearly defined surge of prolactin was not observed. The contribution of low peak LH levels to reduced fertility following progesterone therapy is discussed.