Twenty-five mg. of PGF2 alpha were injected in forty-three lactating cows and eighteen young heifers at ten-day intervals to produce synchronisation of oestrus. Only twenty-four lactating cows (58 per cent) and fifteen young heifers (83 per cent) came on oestrus with two, three or four days after the second injection of prostaglandin. Within eighty hours after the second injection, all animals were inseminated with semen of good quality. This resulted in pregnancy in eight (19 per cent) dairy cows and ten (56 per cent) young heifers. Rectal examination at the time of the second injection revealed a number of changes in the lactating cows. There was undue follicular activity, follicular cysts were observed in several cases and a corpus luteum measuring more than 15 mm. in diameter was not present in 25 per cent of the animals. In addition, twenty-seven cows were synchronised by injection of 0.5 mg. of Estrumate (a synthetic prostaglandin analogue) during the puerperium. Injections were made twenty and thirty days post partum. Eighteen cows (67 per cent) came on oestrus within two, three or four days after the second injection. Insemination which was carried out during oestrus, resulted in pregnancy in five cows. Rectal examination did not reveal any abnormalities in these animals.