During lactation 30 sows were given a basal diet containing 0·39% lysine or the same diet supplemented with 0·1, 0·2, 0·3 or 0·4% Llysine. In addition a group of six sows was given a high protein/high lysine control diet containing 17% crude protein and 1·06% lysine. Piglet weight gain, sow milk yield, milk contents of total solids, protein, fat and energy and efficiencies of utilization of energy and protein for milk production increased progressively as dietary lysine levels were raised to 0·59 %. Sows receiving the lysine-supplemented diets had markedly higher efficiencies of conversion of dietary protein into milk protein and lower efficiencies of energy utilization than those given the high protein/high lysine control diet. Nitrogen retention and biological value of the dietary protein increased quadratically with increasing dietary lysine concentrations, reaching maximum values at 0·59% lysine. Apparent digestibilities of isoleucine, lysine, methionine, threonine and valine in the basal diet were closely correlated with digestibility of nitrogen. Plasma lysine concentrations remained depressed and relatively constant at low dietary levels of lysine, but increased sharply at 0·59 % dietary lysine coinciding with a marked drop in circulating concentrations of other essential amino acids.