The behaviour of sows and their litters housed in different environments of increasing social and physical complexity was observed. Lactational oestrus occurred only in the most complex housing system (4 sows grouped, full boar contact, four indoor pens with four connected outside yards). The amount of time the sows were active did not differ between the housing systems whereas the piglets were significantly more active in the most complex housing system. The amount of social contact between the sows and the piglets was the main factor influenced by the housing conditions. Several measures of social interactions and spatial arrangement consistently showed that social contact was highest in the farrowing crates and lowest in the most complex housing system. The results indicate that increasing the complexity of a housing system for lactating sows reduces the contact between the sow and the piglets which may enhance the incidence of lactational oestrus.