Progressive changes in the composition of milk from rats has been studied from day 0 to 20 of lactation and for 3 days following separation of the dams and pups at day 20 post partum. 2. The changes in concentration of Na, K and lactose suggested that secretion both prepartum and following weaning occurred by a paracellular mechanism whereas a transcellular pathway existed during established lactation. 3. The concentration of total protein and casein increased gradually throughout lactation. In contrast, the concentration of serum albumin increased and transferrin decreased markedly during early lactation. The fat content of milk declined 3-fold within 5 days of birth but the concentration of Ca, Mg and inorganic P increased. The concentration of each of these milk constituents remained constant during established lactation. 4. Following weaning the pronounced decline in lactose, K and inorganic P was negatively correlated with an increase in all other milk constituents except fat. 5. Rats fed a low energy diet produced milk with a lower fat content but with an unaltered concentration of protein and carbohydrate. The growth rate of these litters was similar for the first 5 days of lactation when compared to litters from dams fed a high energy diet. The growth rate of litters thereafter and following weaning was greater for rats fed a high energy diet.
