Trace amounts of vanadium catalyze the oxidation of phenylhydrazine-p-sulfonic acid by sodium chlorate. The diazonium salt produced is coupled with 1-naphthylamine to form an azodyestuff whose absorbance is read at 530 nm. Interferences ate eliminated by separating the vanadium by solvent extraction. Physiological concentrations of vanadium are determined in 10 ml. of ashed human blood or plasma and 25 ml. of ashed human urine. Recovery studies are reported along with results for the analysis of blood and urine samples.