The ability to predict microbial protein yield in any feeding situation is important in maximising the utilisation of feed nitrogen since it will guide decisions as to whether to take steps to improve rumen function or provide supplementary undegradable protein (UDP). The UK Metabolisable Protein (MP) system (Agricultural and Food Research Council, 1992) predicts the yield of microbial protein from the rumen according to the supply of Fermentable Metabolisable Energy (FME). This has led to an improved understanding of responses seen on farms, though there remains uncertainty about microbial protein yield. The excretion of purine derivatives (PD), allantoin and uric acid, in urine has been used as a non-invasive index of microbial protein flow to the duodenum (Chen et al, 1990) and might form the basis of an on-farm diagnostic test. In earlier work, with spot sampling of urine (Dewhurst et al, 1994), we have shown a relationship between milk protein yield and urinary PD excretion; the current work extended these studies by utilising 6-day total collections of urine.