The colorimetric assay for hydroxamic acids (HA) based on diazotation and coupling with α-naphthylamine, has been adapted for automatic determinations using a Technicon AutoAnalyzer. A manifold for discontinuous and continuous assays is described. The sensitivity of the method is very high : aqueous solutions containing HA in amounts as low as 0.5 μg/ml can be easily quantitated. The time for the analysis of one sample varies between 2 to 5 min depending from the accuracy required. The application of the continuous method to drug release problems involving HA is discussed.