Groups of gilts of similar age, weight and breeding were allocated at 165 days of age to four treatments. One group of gilts was isolated from boars throughout the experiment. Fewer animals reached puberty during the experiment in this group than in the other groups where boars were introduced. Introduction of a boar to gilts at 165 or 190 days of age resulted in precipitation of oestrous activity. At 165 days, considerable synchrony of oestrus was obtained when there was a rotation of the boars used for testing, but the effect was much less marked when only one boar was used.