One hundred and eight cows and 65 maiden heifers which were pregnant from 32 to 250 days were treated with 500 meg cloprostenol. Abortions occurred in 53 (49%) cows and 47 (72%) heifers, with 62% of aborted foetuses being expelled within 5 days of treatment. Of 96 pregnancies treated before 150 days, 86 (89.5%) were terminated. Luteolysis, followed by oestrus and ovulation occurred in both those which aborted and those which did not, although serum progesterone levels and ovarian palpations suggest that luteolysis was not complete in those animals which maintained pregnancy. Abortions occurred in only 2 of 51 cows at 151 – 250 days pregnancy, while 12 of 26 similar pregnancies were terminated in heifers. Reasons for this difference are discussed, as are possible methods for improving the success rate in aborting pregnancies between 151 – 250 days.