Recent results in both plant biochemistry and animal metabolism emphasise that, in any research on the nutritional applications of Leucaena leucocephala, measurement of mimosine should be accompanied by measurement of the degradation product 3-hydroxy-4(1H)-pyridone. This may reveal wide differences in the toxic load coming from the feed and in the animal metabolic response to it. The authors suggest that where mimosine analysis can only be performed by the traditional colorimetric method, it should be combined with paper chromatography to estimate ratios of mimosine and 3 ,4-DHP. However high performance liquid chromatography provides vastly improved methods for simultaneously and precisely analysing both compounds. Published methods are reviewed and a new ion exchange method is described which has been used for the rapid routine analysis of a large number of samples. The chromatographic properties and separation of 2,3-dihydroxypyridine, a recently discovered metabolite, and of 3-glucuronyl-4(1H)pyridone from the above compounds are also reported.