Regrouping of sows after weaning can be a very stressful event and this study examined the practice of introducing these sows to each other prior to breeding so that the stress during pregnancy is reduced. Sows were weaned into groups of 8 sows immediately on removal from the farrowing crate. After 4 days the sows were stalled for breeding. Eight groups were reunited into their original groups after 7 days in the stalls, while 9 groups were reunited after 28 days. The aggression that followed the initial introduction was more extensive than either of the reunions, after either 7 or 28 days. Those kept apart for 28 days fought more upon being reunited than those separated for only 7 days. So, it is possible to reduce post-breeding aggression by familiarizing pigs to each other prior to breeding, and by reuniting them after a shorter period of separation for breeding.