The effects of low protein and low Ca intakes during pregnancy on the chemical composition and histological appearance of selected bones of ewes and lambs is reported. Twenty-eight 6£-year-old Blackface ewes were used in a 2 x 2 experiment in which semi-purified diets containing 11-8 and 6-0% crude protein and 1-2 and 0-11% Ca in the dry matter were offered in amounts designed to induce a level of undernutrition typical of that experienced by hill sheep in winter. I n addition, six sheep were killed in early pregnancy as controls, and a further six were offered, during pregnancy, a conventional diet containing 16% crude protein and 1-9% Ca in the dry matter at levels which prevented energy undernutrition. Ewes and lambs were killed at parturition and t h e tibias, 3rd ribs and 3rd and 4th lumbar vertebrae removed for chemical and histological examination.The degree of mineralization of t h e organic matrix (ash:organic-matter ratio) of the bones a t the beginning of pregnancy was poor and it increased in all groups during pregnancy largely irrespective of protein or Ca intake, b u t especially in the wellnourished sheep. The density of ash in the whole bone (ash: volume ratio) increased in the well-nourished and high-protein groups b u t did not change in the low-protein groups. These differences were attributable to t h e effect of protein nutrition on the density of the bone organic matrix (organic matter: volume ratio). Matrix density of the bones of the well-nourished groups increased while that of the low-protein groups decreased.The histological data essentially confirmed this picture and showed that trabecular or cancellous bone from the tibial head and vertebrae of the low-protein and to a lesser extent t h e high-protein sheep was porotic, b u t microscopic assessment conflicted with the chemical findings in that histologically Ca intake appeared to affect bone quality in the vertebrae.There was considerable variation in the histology of individual lamb bones which was independent of maternal nutritional experiences, b u t the overall size of the bones was determined by maternal protein nutrition. TXTrnTJ _ 1 -TT -I T _ . T the protein status of an animal can be as important as mineral intake in determining skeletal mineraliLittle recorded information is available concernzation. ing t h e relative importance of undernutrition andControlled indoor experiments (Sykes & Field, mineral deficiencies in the fluctuations which 1972a, 6) demonstrated the overriding importance occur in t h e mineral content of the skeletons of of protein status compared with Ca intake on ekelebreeding sheep. Evidence from other species tal mineralization during pregnancy at the levels of (Frandsen et al. 1954; Fontaine, Mandel & Gries, energy undernutrition often experienced by sheep 1950;El-Maraghi, Platt & Stewart, 1965) has shown on hill pastures. The present paper provides more that the general level of nutrition and in particular detailed chemical and histological data on t h e 28 A G S 8 I