A dynamic mathematical model of energy and protein metabolism of lactating sows is described in this article. The model is designed to contribute to a systematic and quantitative understanding of the biological connection between diet and reproduction. It traces the flow of energy-containing nutrients from absorption through intermediary metabolism, into and out of body stores, and into milk. State variables (pools) include lysine, other amino acids, glucose, fatty acids, acetate, propionate, acetyl-coenzyme A, ATP, oxygen, carbon dioxide, urea, lean-body protein, visceral protein, storage triacylglycerol, milk protein, milk triacylglycerol, and milk lactose. The rate of each transaction is a function of substrate and inhibitor concentrations, assuming saturable kinetics. Protein and fat turnover, substrate cycles, and the energy cost of membrane transport are explicitly considered. Dynamic simulation of a complete lactation is accomplished by iterating the numerical integration of 16 first-order, nonlinear differential equations, each associated with a pool. Most kinetic parameters were estimated indirectly. Preliminary tests suggest the model is sound. It promises to be useful as a research tool and, with further development, as a guide to practical feeding strategies.