Improvement in the efficiency of milk synthesis in the sow can be accomplished only with a thorough understanding of the metabolic mechanisms relating nutrient intake to lactational performance. We have begun to describe these connections systematically and quantitatively through mathematical modelling. Our model is an integration of our present knowledge base with concepts of lactating sow metabolism. It is limited by serious inadequacy of present information, but it has guided us to ask key questions which can be investigated by animal experimentation. We are presently studying the metabolism of lipids in adipose tissue, and the impacts of lysine and amino acid intake on metabolic state and lactational performance, to elucidate aspects of the connection between nutrient intake and lactational performance. The pressing need for more information at several levels of biological organization is described. Collaboration among scientists across disciplines can increase the productivity and efficiency of both scientific and practical endeavours. It is imperative that publication of research results include thorough descriptions of animals, diets and environments to allow better interpretation and use of results.