Ninety eight lactating Bonsmara cows were divided into an experimental group of 66 and a control group of 32 cows. The 66 cows were synchronized by means of the interrupted method of progesterone administration and thereafter inseminated during the first two grouped heat periods. The 32 control cows were observed for signs of oestrus and inseminated after the beginning of the progesterone treatment started in the experimental group. After the period of treatment the cumulative percentage of cows that had exhibited oestrus at any stage was very similar in the two groups. Likewise the cumulative conception rate at any stage was also alike in both groups. Although the insemination dates of the synchronized cows were concentrated during two short periods, their calving dates were no better synchronized than those of the control cows. It is concluded that neither the number of cows that exhibit oestrus nor the number of conceptions are affected by the synchronization treatment. It appears that synchronization of ovulation in beef cows is useful only to group the cows’ heat periods for solving some of the managerial problems associated with observation for oestrus under extensive conditions.