Problems of milk protein production and the effects of diet on milk secretion are considered. The process of digestion of dietary nutrients, and the hydrolysis of proteins to amino acids are described. Microbial fermentation in the digestive system of the cow and the effects on the mixture of nutrients derived from the food are shown in a table. The utilization of substrates in the udder and the mechanism for protein synthesis are assessed and a summary showing the outlines of mammary metabolism given. The paper discusses feeding under farm conditions in relation to energy nutrition and the importance of promoting milk secretion in early lactation is stressed. (Editor's summary)