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1. Changes in milk composition and in mammary permeability to labelled sucrose and monovalent ions have been studied in late pregnancy and at the time of parturition. These data have been compared with those obtained previously in lactating goats.

2. Colostrum contained more sodium, chloride, protein, immunoglobulins and less potassium and lactose than milk. The composition of the aqueous phase changed near term and normal milk composition was reached either several days before or several days after parturition. The main decrease in the immunoglobulin concentration occurred when the animal was first milked or suckled.

3. In contrast to lactating animals, [¹⁴C]sucrose passed from blood to milk and [¹⁴C]lactose from milk to blood. There was a significant correlation between the entry of labelled sucrose into milk and the sodium concentration.

4. The rate of passage of ²⁴Na and ³⁶Cl into milk from blood was higher in late pregnancy than in lactation.

5. In late pregnancy, the concentrations of both sodium and chloride were inversely correlated with the concentration of lactose while there was a positive correlation between potassium and lactose.

6. Hydrostatic pressure in the lumen of the mammary gland increased during late pregnancy but there was no significant correlation with the composition of the aqueous phase of milk.

7. When one gland of 4 animals was milked twice-daily starting 9-19 days before parturition, the composition of the secretion changed to that of normal milk only in the gland that was being milked. Similarly, [¹⁴C]sucrose, ²⁴Na and ³⁶Cl entry from blood decreased. Milk yield also increased but a further increase occurred post-partum.

8. Comparison of the passage of ³HOH, ²⁴Na and ³⁶Cl into different fractions of milk during I.A. infusions indicates that the mammary ducts are less permeable to ions than the alveoli, but it was not possible to decide whether the ducts are as impermeable as during lactation.

9. It is suggested that ion and lactose movements occur via a para-cellular route and these results are discussed in relation to transport mechanisms and in terms of local and systemic controlling mechanisms.

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