Our ultimate goal is to understand the mechanisms involved in the regulation of human milk secretion. Descriptive studies focusing on the onset of lactation will be designed to determine whether age, parity, reproductive interval, ethnic origin, diet or maternal disease have an effect on the onset of lactation in women. The volume of milk secretion and milk composition will be measured in a defined population of about 225 women on days 4, 8, 14 and 60 of lactation. This study will provide baseline data for both the testing of hypotheses about the physiological regulation of lactation and for the clinical evaluation of lactational performance. Experimental studies will be designed to determine whether insulin, plasma glucose, cortisol and prolactin are involved in the minute-to-minute regulation of milk secretion. Two protocols will be used: (1) Diurnal variation in milk secretion rates and composition will be correlated with blood hormone levels under control conditions and conditions designed to alter the normal diurnal variation in these hormones. (2) Glucose clamp studies will be used to examine the effects of insulin and glucose on milk production under highly controlled conditions. Clinical studies will continue to evaluate the causes of lactation failure. Compositional variables currently measured include milk concentrations of lactose, glucose, lipid, total protein, Na, K, Cl, free phosphate, citrate, calcium, Mg, pH, pCO2, urea, creatinine, Zn, Cu, Mn, and Cr. Analyses will be expanded to include milk amino acids, the major milk-specific proteins, lactoferrin, Alpha-lactabumin, casein and IgA and the trace elements Fe, Ni, Mo, and Se. A "reference library" of well-characterized milk samples will be maintained to be used by investigators in other laboratories for analysis of other milk components. These studies should allow improved recommendations about infant feeding to be formulated, based on more complete understanding of the maternal physiologic factors which regulate human milk production.