A series of experiments was carried out to study the factors affecting faecal endogenous P excretion in sheep. Experiments 3.1, 3.2 and 3.3 involved intensive balance measurements and radio-isotope (<SUP>32</SUP>P) dilution measurements made on 6 mature sheep, to study the effect of changing salivary secretion on P metabolism. A hay diet (P intake 2.2 g/d) was fed in 3 different physical forms (finely ground, coarsely ground and chopped) in experiment 3.1 and a low-P oat husk based diet was given in coarse or finely ground form in experiment 3.2. The results of both the experiments indicated that there was no change in salivary P flow, faecal endogenous P excretion and P balance within each experiment. In experiment 3.3 sheep were fed different amounts of a low-P (P intake 1.2 g/d) oat husk based diet (0.8, 1.1 and 1.4 kg/d). There were no significant changes in salivary P flow or balance position. However, in this experiment there was an increase in faecal endogenous P excretion but the size of this increase with increase in dry matter intake was less than that reported by TCORN (1989). This increase was attributed to increased secretion of P in the intestinal secretions in response to an increase in dry matter flow through the intestine. Experiments described in Chapter 4 were performed to explore the possibility of using sheep maintained by total intra-gastric infusion to measure minimum endogenous excretion of P in faeces and urine. The results indicated that sheep given a near zero P intake excrete amounts of P that are comparable with the levels predicted by ARC (1980) as minimum endogenous P excretion. Chapter 5 describes experiments carried out to investigate factors affecting intestinal P absorption in the sheep. Experiment 5.1 was aimed at studying the effect of increasing demand for P, by intravenous infusion of CaCl<SUB>2</SUB>, on intestinal P absorption. The results of this experiment showed that while the amount of dietary P absorbed is mainly determined by P intake there is a component of this system that is sensitive to changes in P requirement. There was an increase in intestinal P absorption but no change in endogenous P secretion into the gut. Three sheep surgically prepared to isolate a one-metre section of proximal small intestine were used in experiments 5.2 and 5.3. In the Experiment 5.2 sheep were given a continuous intra-venous infusion of a buffered solution of P (Na<SUB>2</SUB>HPO<SUB>4</SUB>NaH<SUB>2</SUB>PO<SUB>4</SUB>, 3g of P/d) and the loops were perfused with a solution (pH 4.5, Tonicity 280 mOsmol/litre) containing 30 mmol/litre of P. The results showed that the amount absorbed was much less during intravenous P loading. Experiment 5.3 was performed to study the absorption of P from the intestinal loops when they were perfused with solutions containing a fixed quantity of P at varying P concentrations (20 mmol/litre and 30 mmol/litre). There were no significant differences seen either in the total amount or in the absorptive efficiency between treatments. A lactation study (described in Chapter 6) was carried out to investigate the effect of feeding a high protein diet on Ca and P metabolism. Measurements were made on two groups of six ewes with one group being restricted in protein supply while the other group was given supplementary protein in the form of blood meal. Balance measurements were made over 10-day periods separated by 10-day rest periods. <SUP>45</SUP>Ca and <SUP>32</SUP>P were used to measure faecal endogenous Ca and P losses. An extra balance experiment was carried out on the second set of ewes after they ceased producing milk. The results indicated that the loss of mineral from the skeleton in ewes in early lactation is proportional to milk production and is not directly dependent on dietary protein supply or N balance. The results also suggested that the rate of repair of bone matrix and its mineralization during the post lactation period is affected by protein supplementation. Two experiments (Experiments 7.1 and 7.2) were carried out to study the effects of high dietary Ca:P ratio on faecal endogenous faecal excretion in mature sheep which were given sufficient P to meet their maintenance requirement according to ARC (1980). The results obtained showed that faecal endogenous P excretion was not significantly affected by feeding diets high in Ca. All animals had a level of salivary P secretion that was more than adequate to meet rumen microbial requirements. Experiments 7.3 was a slaughter study carried out to investigate the effects of an increase in Ca intake on growth of the young lambs fed on diets supplying just sufficient P to meet their estimated requirements based on the revised TCORN (1989) growth model. A total of 40 lambs in four treatment groups were used in this study. The results of this trial showed that feeding high levels of Ca had no adverse effects on growth of the lambs when P was adequate . There was no beneficial effect of feeding excessive Ca and P but there was an increase in the incidence of urinary calculi. The amounts of Mg retained by the lambs were lower than the levels on which ARC (1980) based their estimates of requirements.