A method is described for the estimation of kynurenine in plasma via alkaline cleavage, extraction and diazotisation and coupling. Tryptophan interference, which is demonstrated in an earlier method, has been eliminated. Data on the specificity of the present method and the normal adult male range of fasting plasma kynurenine are presented. The time course of plasma tryptophan and kynurenine after a tryptophan load are studied in 3 subjects as an additional validation of the method, and as an example of its application. The method described is convenient and requires no complex apparatus.