Studies were undertaken to determine the progressive changes and relationships between the major constituents in the mammary secretion of breast feeding and non-breast feeding women during the initiation of lactation. The concentration of metabolites (lactose, glucose and urea), electrolytes and proteins (total protein, alpha-lactalbumin, lactoferrin, albumin, IgA, IgG and IgM) were measured in small samples of mammary secretion (0 . 5-5 . 0 ml). Colostrum during late pregnancy contained higher concentrations of proteins and lower concentrations of metabolites than milk in established lactation. Of the electrolytes, the concentrations of sodium, chloride and magnesium were higher, whereas potassium and calcium were lower in colostrum than in milk. The osmolality of the secretion remained relatively constant over the pre-partum and post-partum period. These findings showed that the initiation of lactation developed in two phases, first a limited secretion of milk constituents in late pregnancy and then true induction of lactation (lactogenesis) 32-40 h after delivery. The changes in the mammary secretion of non-breast feeding women during the first 3 days post-partum were similar to those observed in breast feeding women but reversed abruptly during the next 6 days, indicating the onset of mammary involution. This finding demonstrated that breast feeding is not a major factor for the initiation of lactation but is essential for the continuation of full lactation.