N-Hydroxycarbamates, mono-hydroxyureas and di-hydroxyureas in tissue extracts have been determined by diazotising sulphanilamide with the nitrite produced on oxidation and coupling with N-1-naphthylethylenediamine. Mixtures of hydroxylamine and hydroxamic acids were determined (a), by selective oxidation at pH 3·5 and pH 8·0, and (b), after separation of the components by thin-layer chromatography.