Although human milk is acknowledged to be the ideal food for the infant, little information is available concerning the infant's use of specific functional components or the dynamics of milk production by the mother. The evaluation of lactation performance is based on 3 types of criteria, infant outcome and the quality and quantity of milk produced. To date the assessment of infant outcome generally has been based on anthropometric standards or on the occurrence of morbidity and mortality; milk quality on the concentrations of proteins, fats, and minerals; and that of milk quantity on production volumes and concentrations. Within the contexts of these assessments associations have been sought most commonly between lactation performance and maternal age, maternal nutritional status, length of pregnancy, duration of lactation, lactation management protocols, and lactation and reproductive histories. Efforts to identify the determinants of lactation performance have been hampered by wide interindividual differences in milk composition. The basis of these differences and their effects on the infant are understood poorly. Yet, that information will be important in understanding the genesis of successful or failed lactation and the effects on the nursing infant. Long-term objectives of this proposal are to develop more specific assessments of infant outcome and to describe the physiologic basis for the marked interindividual differences in milk composition and production volume. Specific aims are to determine if selected milk immunologic factors are absorbed by the infant or if endogenous production is stimulated and to assess endogenous losses of index amino acids as a measure of gastrointentinal maturity. Mothers of premature infants will be infused with 13C-leucine and 15N-lysine to label human milk endogenously. Balances will be performed in infants fed endogenously-labeled milk to differentiate the passive survival or absorption of selected human milk factors from their production by the infnat. Tracer methodologies also will be used to determine relationships between kinetic parameters underlying milk quality and quantity and specific endocrine responses and physical stimulation to the breast. 12-hr. infusions with labeled amino acids will be performed on mothers of premature infants during the period of lactation matintenance. Infusions will be performed during control periods and during periods of insulin or growth hormone stimulation. Control studies are planned during the periods of lactation initiation and establishment. Specified diets and lactation management protocols will be followed. Kinetic parameters will be measured during periods of insulin and growth hormone stimulation.