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 (Ca(tot), Ca(d), Ca2+), diffusible phosphate (Pi(d)). diffusible citrate (Cit(d)) and lactose in the breast milk. On day 1 after birth the concentrations (mean +/- SEM) were Ca(tot), 5.71 +/- 0.30 mM: Ca(d), 2.66 +/- 0.19 mM; Ca2+, 2.90 +/- 0.18 mM; Pi(d), 0.26 +/- 0.16 mM; Cit(d), 0.25 +/- 0.03 mM and lactose. 76 +/- 11 mm. Between day 1 and day 4 the concentration of Ca(tot) increased 1.7-fold to 9.56 +/- 0.39 mM, Ca(d) increased 1.8-fold to 4.75 +/- 0.26 mM, Ca2+ decreased by 20% to 2.33 +/- 0.13 mM. Pi(d) increased 6.6-fold to 1.69 +/- 0.11 mM, Cit(d) 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 [Ca(d)] and [Cit(d)] in the milk of both ruminant and non-ruminant species, which show a wide range in concentrations of [Ca(d)] and [Cit(d)], 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 (Ca(d)] and [Cit(d)] in human milk. even during lactogenesis II when the composition of the milk is changing very rapidly.