A.R.C. Institute ofAnimal Physiology, Babraham, Cambridge, CB2 4AT (Received 23 May 1975) In the rabbit after about day 18 of lactation, while milk yield remains relatively high, milk [Na], [Cl], [fat] and [protein] increase and [K] and [lactose] decrease; these changes can be reversed by exogenous prolactin (see Linzell & Peaker, 1971). Recently it has been proposed that the change in the composition of the aqueous phase of milk can be attributed to an increase in the paracellular movement of ions and small molecules between extracellular fluid and milk probably through leaky 'tight junctions', and that prolactin in some way affects the permeability of this pathway (Taylor, Peaker & Linzell, 1975; Linzell, Peaker & Taylor, 1976; Peaker & Taylor, 1976). In the guinea-pig, Mepham & Beck (1973) found that during lactation, milk [K] decreased while [Na] increased; [lactose] decreased but recovered to some extent by day 14. The present observations