I.N.R.A., Theix, 63110 Beaumont, France and Groupe Hospitalier de la Timone, 13385 Marseille, France (Received 31 May 1978) Very little information is available concerning interrelationships between maternal and foetal concentrations of 25-hydroxy vitamin D3 (25-OHD3; Hillman & Haddad, 1974; Ross, Care, Pickard, Peacock & Robinson, 1976; Weisman, Sapir, Harell & Edelstein, 1976). This paper describes the concentrations of 25-OHD3 in the plasma of pregnant and lactating ewes and their foetal and newborn lambs. Five ewes bearing single lambs were selected by radiography performed on day 100 of gestation in winter. The daily intakes of calcium, phosphate and magnesium for each animal were respectively, 11, 7 and 2 g. From day 26 pre partum until day 4 post partum, blood samples were collected through catheters implanted on day 115 of gestation in the left foetal and maternal carotid arteries (Mellor & Matheson, 1975). Plasma concentrations of 25-OHD3