A total of 273 Hereford cross heifers were treated with intravaginal progestagen pessaries to determine some of the factors affecting oestrous response and fertility following long-term (20-day) and short-term (10-day) treatments. Oestrous response and degree of synchronization were high after treatment for 20 days, but the fertility rate was lower than that of control heifers. There was no difference in the fertility of heifers inseminated artificially and those mated naturally. When the treatment period was reduced to 10 days and 900 mg progesterone and 5 mg oestradiol valerate given intramuscularly at the start, a high oestrous response and a low degree of synchronization resulted, but the conception rate was similar to that of the control animals. Reducing the dose of progesterone to 250 mg resulted in a high oestrous response and a high degree of synchronization. The stage of the cycle at the start of the 10-day treatment did not affect the oestrous response. Retention of the progesterone pessary was low (79·9%) in heifers treated for 20 days, but was 100% in those treated for 10 days.