The onset of copious milk secretion (lactogenesis II) in women occurs between 1 and 3 d after birth, and during this period the composition of breast milk changes. During the first 5 d of lactation we measured the concentrations of total, diffusible and ionized Ca (Catot, Cad, Ca2+), diffusible phosphate (Pid), diffusible citrate (Citd) and lactose in the breast milk. On day 1 after birth the concentrations (mean ± SEM) were Catot, 5·71±0·30 mM; Cad, 2·66±0·19 mM; Ca2+, 2·90 ±0·18 mM; Pid, 0·26±0·16 mM; Citd, 0·25±0·03 mai and lactose, 76±11 mM. Between day 1 and day 4 the concentration of Catot increased 1·7-fold to 9·56 ±0·39 mai, Cad increased 1·8-fold to 4·75±0·26 mM, Ca2+ decreased by 20% to 2·33 ± 0·13 mM, Pid increased 6·6-fold to 1·69±0·11 mM, Citd increased 20-fold to 5·06±0·21 mM, and lactose increased 2·3-fold to 173±4 mM. A high correlation has been found between [Cad] and [Citd] in the milk of both ruminant and non-ruminant species, which show a wide range in concentrations of [Cad] and [Citd], and the data fit a simple physicochemical model of ion equilibria in the aqueous phase of milk. The results of the present study confirm the relationship between [Cad] and [Citd] in human milk, even during lactogenesis II when the composition of the milk is changing very rapidly.