Although lactation is an essential component of mammalian reproduction, each species has evolved unique lactation strategies that optimise the growth and development of their young. However, these evolutionary strategies have imposed physiological and environmental contradictions in commercial pig production. In the sow only extremes of either under-or over-nutrition have been reported to influence mammogenesis and lactation, and the incompatibility between the zones of thermal comfort for piglets and sows often results in ambient temperatures in commercial piggeries that impair either the growth rate of piglets or milk production in sows. Of the factors investigated to date, litter size has the greatest positive influence on total production of milk from sows. This observation strongly suggests that milk production is regulated at the level of each mammary gland (e.g., autocrine control) rather than systemic metabolic processes (e.g., hormonal control). While the short period of milk flow (10 to 20 s) during milk ejection could facilitate survival of larger litters, the restriction of milk intake by the piglet due to this short period of milk flow, and the local inhibition of milk synthesis by autocrine control mechanisms, may be important rate-limiting steps restricting potential milk production in domesticated sows. Although the very tight control of milk flow imposed by the milk-ejection reflex and the ambient temperature requirements of piglets could have provided an evolutionary advantage to larger litters, these factors may limit the expression of the domesticated sow's capacity to produce milk as well as her piglets' potential for growth and development. It is important to determine the significance of these subtle influences on pig production so that both experimental protocols and management practices do not compromise expression of the full lactation potential of the sow. (C) 1997 Elsevier Science B.V.