Thirty large white cross Landrace prepubertal gilts from five litters were allocated at random within litters to one of six treatment. All the gilts were given three daily intramuscular injections of 6 micrograms/kg liveweight of oestradiol benzoate but the first injection was given at either 100 days, 120 days, 140 days, 160 days, 180 days or 200 days old. From the time the first oestrogen injection was given, until the end of the experiment, the gilts were allowed daily contact with a boar for 20 minutes. The gilts were mated with a mature boar at the appearance of the second oestrus and they were slaughtered 20 days later. The numbers of gilts ovulating in the treatment groups were determined from plasma progesterone concentrations measured twice weekly throughout the experiment. The numbers of gilts ovulating within 10 days of the oestradiol benzoate injections were 3 of 5, 1 of 5, 0, 0, 3 of 5 and 1 of 5, respectively, in the groups first treated at 100 days, 120 days, 140 days, 160 days, 180 days and 200 days old. For those gilts which were successfully mated, no significant differences were observed between the treatment groups in the number of viable embryos per gilt.