The suckling behaviour, the development of daily weight gains and biochemical blood parameters (immunoglobulin G, total protein, glucose, urea) were investigated during the suckling period (35 days) of piglets (Sus scrofa) in a group housing system for nursing sows (group P: 4 primiparous sows, 34 piglets; group M: 4 multiparous sows, 43 piglets). Following an imprinting period of 10 days, piglets were given the opportunity to leave the farrowing pen and to interact with the other sows (variant e: suckling bouts at own mother only, variant f: suckling bouts also at one or more unfamiliar sows). Although a number of piglets suckled at unfamiliar sows (Pf = 73.5%; Mf = 32.5%; p = 0.01), only those piglets that stayed with their own mother after mixing had some "physiological advantages" (Pe: daily weight gain from day 10 to 35, p = 0.05; Me: IgG, p = 0.05). Piglets of primiparous sows had a lower teat pair fidelity within the imprinting period (day 1 to 10). During this time, piglets of both groups suckling at unfamiliar sows showed a lower teat pair fidelity (P: ns; M: p = 0.05), tended to prefer the posterior part of the udder, and their blood glucose and urea contents were higher (P: ns; M: p = 0.05). This was interpreted as an indication of competitive strain which obviously encouraged the piglets to leave their mothers. It is necessary to carry out further systematical studies on sociophysiological mechanisms.