PGF2alpha either as a subcutaneous injection (8 or 10mg) or intrauterine infusion (dose 5 mg) successfully synchronised oestrus in beef cattle AI programs. The beef cattle programs were managed by 1 of the following 2 systems. 1. Two 10-day insemination periods, 1 for the first and the second for the repeat inseminations. Oestrus was detected and oestrus cows inseminated for the first 4 days of the first period. On the fifth day all cows not inseminated were treated with PGF2alpha. The treated cattle came into oestrus and were inseminated over the following 2 to 5 days. 2. Two PGF2alpha treatments 10 days appart followed by two 4-day insemination perids. During the first 4-day period, cows exhibiting synchronised oestrus were inseminated. Cows returning to oestrus were inseminated during the second 4-day period, about 18 days after the first, period. Synchronisation of oestrus was not sharp, oestrus being distributed over 7 days, with the majority of cows being in oestrus on the third (42.3%) and fourth (22.4%) days. The route of administration of PGF2alpha (injection or infusion) did not effect oestrus synchronisation. PGF2alpha by either route of administration did not effect the fertility of cows inseminated at either the synchronised or subsequent oestrus. Pregnancy rates at the synchronised oestrus were higher when cows were inseminated after dectection of oestrus than when they were inseminated without reference to oestrus at a fixed time 3 days after PGF2alpha treatment (mass injection). Oestrus synchronisation greatly reduced the input of time, labour and feed required during the total program. However, oestrus synchronisation did not alter the overall efficiency of artificial breeding programs measured in terms of the proportion of cattle entering a program that were inseminated and became pregnant. The main problem encountered was drug wastage through unknowingly treating cows and heifers that were not cycling.