The concentrations of diffusible Ca and Mg in milk diffusates from 3 goats were determined before, during and after a 2-d period of starvation. Free Ca-concentrations were determined by a murexide method and compared with values calculated from a detailed model of the ion equilibria. It was shown that diffusible Ca increased as citrate increased during fasting and both decreased after refeeding, but there was a difference of about half a day in the response curves, with citrate following Ca. The difference was accounted for mainly by a change in free Ca2+ concentration which in turn was related mainly to changes in milk pH. Diffusible Mg underwent changes similar to those of diffusible Ca.