The effect of progesterone synchronisation on the plasma progesterone and luteinizing hormone (LH) concentrations in sheep were studied. During their normal breeding season 10 Merino ewes were injected with l0 mg progesterone daily for 15 days to synchronise their oestrous periods. Ten ewes served as controls. Blood samples were collected daily during the progesterone treatment period and then at 6 intervals up to the end of oestrus. The plasma was analysed for progesterone and LH. A close relationship was found between the LH peak and the end of oestrus in control sheep. This relationship was often disrupted by the progesterone treatment. No explanation for the shift in the LH peak could be found in the preceding progesterone changes in both treated and control animals.