Abnormal behaviour patterns were observed in piglets weaned from their mother 1 day after birth and reared artificially in small individual incubators. The incubators were plastic bins, 0.057 m3 in size, with a window at the front through which a nipple-drinker provided feeds of Carnation milk at hourly intervals. Piglets were kept in the incubators until they were 18 days old, when they were placed in pairs in 1 m2 flat deck cages. Pairs of piglets reared by their mother to this age were also introduced to the flat deck cages to act as controls. The results after a week in the incubators showed that screaming and whining occurred at abnormally high rates in the incubators, compared with in a reference environment, as did lying and sitting. Walking and scampering were performed at abnormally low rates. Nosing and chewing at the forelegs occurred at a rate of 6.32 times per hour in the incubators but never in the reference environment. Massaging at the udder of the sow was redirected towards the area of the window around the nipple-drinker in the incubators and was performed at a much higher rate than massaging of the udder. When transferred to the flat deck cages, the incubator-reared piglets showed a very high rate of massaging, sucking and up the body of their partner, behaviour rare in the sow-reared piglets. After 10 days in the flat decks, both groups showed significantly high levels of massaging, sucking and levering. Differences between the incubator-reared and sow-reared piglets in the flat decks included the development, in the incubator-reared piglets, of massaging the wall behind the water nipple, and of frequently running back and forth between the feed trough and water nipple when fed, habits not seen in the sow-reared piglets. ADDITIONAL ABSTRACT: Behaviour patterns were studied in piglets reared artificially in incubators of 0.057 msuperscript 3 capacity until 18 days old, and then placed in pairs in flat deck cages (1 msuperscript 2). Abnormal behaviour included a high prevalence of screaming, whining, lying and sitting, with little walking and scampering; nosing and chewing forelegs; massaging of window around the nipple drinker. When transferred to the flat-deck cages there was much more massaging, sucking and levering up the body of the partner than in naturally reared piglets. It is concluded that absence of key stimuli and inability to perform full behavioural repertoire created stress in the piglets, which resulted in the appearance of abnormal behaviour patterns.