Progesterone and progestagens are widely used to synchronise oestrus in sheep, however 15 to 30 % of ewes fail to maintain a pregnancy following the first service after oestrous synchronisation and the reason(s) for this failure rate is unclear. In commercial flocks, the progestagen treatment commences at a random stage of the oestrous cycle and in some ewes luteolysis may occur soon after treatment commences, leaving the ewe solely dependent on the exogenous source of progestagen which promotes the development of persistent follicles (Flynn et al., 2000). These persistent follicles may be up to 14 days of age when they ovulate whereas the age of follicles at ovulation in natural cycles is 4 to 8 days. In cattle, it is clear that ovulation of these old follicles results in a decrease in fertility. The aim of this study was to examine the effect of ovulation of aged follicles on fertility in cyclic ewes by using treatments based on a 14-day, progestagen synchronisation protocol that is known to produce persistent follicles.