The grazing dairy cow is at risk of sodium (Na) deficiency because of the large output of Na in the milk and low sodium concentration in forages (Chiy and Phillips, 1995). During mastitis plasma containing Na, Cl, immunoglobulins and other blood proteins leaks into the mammary gland and secretion declines, thereby reducing K and lactose concentrations in the milk (Schar and Funk, 1986). The Na content of mastitic milk is usually increased by 150-300%, which helps to limit the growth of bacteria, such as Streptococci, that are sensitive to the Na concentration of their growth medium (Skarvdova, 1989). Such an increase in Na output could easily create a negative Na balance and, as the cow has negligible reserves of this mineral, effective control of the intramammary infection may depend on adequate Na status (Janota-Bassalik et al., 1985).