Total P in colostrum of mares rose to a peak 24 h after foaling and then declined with the change to normal milk and throughout lactation; the peak of P in colostrum coincided with the lowest inorganic P values in serum after foaling. Colostrum before the foal sucked contained 17 or 18% total protein, which fell to 2.6% in 24 h and thereafter declined slightly as lactation progressed.