Metabolism of urea in sheep.

4. Plasma concentrations and urea entry rates were significantlj and linearly related. 5. The relationship between excretion rate and plasma urea concentration was best described by a cubic equation. 6. Degradation of urea in sheep was found to be extensive in all the animals studied ; as the protein intake increased, the quantity of urea degraded also increased but the percentage of ureaentering the body pool thatwas degraded was decreased. Animals given a ration containing 3.5 yo crude protein degraded 76-92 yo of the urea entering the body pool. 7. A rectilinear relationship was found between pool size and plasma urea concentration. The urea space in animals given low-protein rations was significantly less than in animals on highprotein rations. 8. The effects of starvation for 2, 4 and 6 days on urea metabolism in sheep were investigated. In a11 the sheep starved for 2 days there was a significant increase in urea pool size, but the entry rate was markedly depressed indicating a retention of urea in the body pool on starvation. 9. A significant amount of nitrogen was found to g o through the system: rumen ammonia-+ portal blood ammonia-tblood urea-wumen ammonia.