The reproductive efficiencies of ewes treated with two injections of prostaglandin (cloprostenol) administered at intervals of either 9, 10 or 11 days, were compared.  The conception rate of ewes in these treatment groups was 11,1%, 40,0% and 70,0%, respectively and that of the control group 82,4%.  These differences indicate the importance of injecting cloprostenol at an interval of at least 11 days.