The nest building behaviour of 12 preparturient sows was studied in enriched pens. Four categories of behaviour were observed: rooting, straw carrying, pawing and lying. In the 24 h before farrowing, six sows excavated hollows in sand floors and removed straw from dispensers to construct nests similar to those reported for sows kept outdoors. When pre-formed nests were presented to six sows on the day before farrowing, straw removal was reduced (P<0.05), straw carrying and pawing remained the same. Rooting, lying and the duration of nest building increased (P<0.05). Nest building is highly motivated behaviour in sows and the performance of the activities themselves appears to have a significant role in reducing the motivation. Nesting material appears to be a key factor both in the regulation and performance of the behaviour.