Sheep grazing heather hills in winter normally ingest a mixture of heather and indigenous herbage, predominantly Agrostis and Festuca spp. Under these conditions during mid-pregnancy, ewes at high stocking rates may lose excessive live weight leading to poor subsequent lambing performance. This undernutrition may be attributed, in part, to low voluntary intakes of herbage and, possibly, to insufficient amino acids being absorbed from the small intestine, since the availability of nitrogen (N) of heather is very low, both in the rumen (Milne, Christie and Russel, 1979) and in the small intestine (MacRae, Milne, Wilson and Spence, 1979). Supplementation could increase the nutrient supply to ewes grazing predominantly heather, since voluntary intake of a heather: Agrostis/Festuca (⅔:⅓) diet was not depressed when a urea and starch supplement was fed (J. A. Milne, A. M. Spence and H. A. Fisher, unpublished). The following experiment was conducted to examine the effects of such a supplement on ruminal digestion and the absorption of non-ammonia nitrogen (NAN) and organic matter (OM) from the small intestine and hind gut.