Observations in Edinburgh on Large White domestic pigs kept in groups outside or in family pens have indicated that sows regularly exhibit lactational oestrus in rich social and physical environments. In the pig park enclosure oestrus occured in 20 of 25 lactations and in all 21 lactations in the enriched family pen system. To further assess the stimulatory importance of selected factors of these environments, lactating sows were housed on three treatment levels differing in environmental complexity: i) conventional farrowing crates, ii) larger pens with optional outside yards, iii) enriched family pens. On each housing level additional treatment factors were applied: locomotory or manipulative possibilities, partial or full social contact with other sows and boars, restriction of suckling or piglet contact.