Over a two year period, 436 suckling beef cows were treated either with a progesterone releasing intravaginal device (PRID) for 10 days, or two injections of 0.25 mg gonadotrophin releasing (gn-RH) at an interval of 10 days, or left as controls. Treatment commenced between days 20 and 40 post partum and cows were subsequently mated naturally at oestrus. PRID treated cows showed a more synchronous onset of oestrus than the control or Gn-RH treated cows. There was no overall significant effect of either treatment on the mean calving interval, although in the first year PRID treatment before day 30 post partum reduced the mean calving interval by 12.8 days compared to the controls (P less than 0.05). Neither bodyweight nor body condition score had a significant effect on the calving interval.