1. A low-sodium diet was given to Blackface ewes over two reproductive seasons. This diet provided a total of 3–7 mmol/d except during lactation when the intake was about 11 mmol/d. Control ewes were given the same diet supplemented with sodium chloride to provide recommended levels which were about tenfold that of the experimental diet.
2. The output of Na in urine and faeces from ewes given the low-Na diet was very low, about 3 mmol/d, from early in the experiment and continued at about this level throughout.
3. Lambs born to low-Na ewes and given a low-Na diet similar to that of their dams during lactation, grew, after weaning, more slowly than corresponding lambs from control ewes, but at 6 months of age when six from each group were killed, tissue Na contents were only slightly lower in experimental than control lambs.
4. Fluids and tissues obtained at the end of the second lactation from four ewes of each group that had suckled twin lambs, were analysed for Na and potassium. The Na concentration in saliva and rumen fluid of low-Na ewes was about half that of control ewes and there were corresponding increases in K: the differences were significant. Although Na concentrations for experimental ewes were generally lower than those for control ewes in the tissues analysed (liver, kidney, heart, brain and bone), the difference was significant only for the heart (P < 0.05). Haemoglobin and packed cell volume were significantly elevated in low-Na ewes (P < 0.01), indicating decreased body fluid volume. Enlargement of the adrenal glands in low-Na ewes (P < 0.01) and specifically in the zona glomerulosa (P < 0.001), showed the marked hormonal response to Na status of experimental ewes given a very-low-Na diet over two reproductive cycles.