Mature sheep fitted with rumen and duodenal cannulae and fed either pelleted hay or grass diets were given supplementary phosphorus by continuous intravenous infusion and the effects on salivary phosphorus secretion, intestinal phosphorus absorption and pathway of excretion were studied. In control periods little phosphorus was excreted in the urine, the faeces being the major pathway for excretion. Infusion of phosphorus increased both urine and faecal phosphorus excretion though in sheep fed the hay diet the urine response was much less marked than in those fed the grass diet. This was not due to differences in plasma phosphate levels but rather to differences in renal tubular reabsorptive efficiency, though the reason for this is at present unclear. Intravenous phosphate loading had little effect on salivary phosphorus secretion, most of the increase in faecal phosphorus excretion being the result of reduced reabsorption within the intestine. The significance of these changes in relation to the control of phosphorus balance in ruminants is discussed.