Abstract: Microgram amounts of isonicotinic acid hydrazide (isoniazid) have been determined on a repetitive basis by sample injection into a continuously recirculated reagent reservoir solution. Chemically, the method makes use of two competitive reactions: (a) formation, in phosphoric acid medium, of a colored complex between isoniazid and vanadium(V), (b) destructive oxidation of this complex by excess of the VO3- reagent. The rates of these two reactions are adjusted to obtain a transient slgnal whose height is directly proportional to the amount of isoniazid In the injected sample.